Add docs for build strategy stuff

Signed-off-by: Nikolas Skoufis <nskoufis@seek.com.au>

docs/features/techdocs/concepts.md

@@ -46,6 +46,24 @@ between `techdocs-backend` and the storage)
 
 [TechDocs Backend](https://github.com/backstage/backstage/tree/master/plugins/techdocs-backend)
 
+## TechDocs Build Strategy
+
+To accommodate more complex logic surrounding whether or not to build TechDocs, the TechDocs backend
+supports selecting a Build Strategy.
+The Build Strategy is responsible for deciding whether the documentation requested should be built locally
+by the TechDocs backend or not.
+Customization of the Build Strategy allows for more complex behaviour regarding whether the TechDocs backend
+is responsible for building TechDocs, whether an external process is responsible, or whether a combination
+of local builds and an external process is responsible, on an entity-by-entity basis.
+
+The default Build Strategy results in the TechDocs backend building documentation locally if the
+`techdocs.builder` configuration option is set to `'local'`, and skipping any building otherwise.
+However any logic that satisfies the Build Strategy interface can be implemented, using the Backstage
+config as well as the entity being processed to make a decision.
+
+For an example of how the Build Strategy can be used to implement a 'hybrid' build model, refer to
+the [How to implement a hybrid build strategy](./how-to-guides#how-to-implement-a-hybrid-build-strategy) guide.
+
 ## TechDocs Container
 
 The TechDocs container is a Docker container available at

docs/features/techdocs/configuration.md

@@ -38,11 +38,16 @@ techdocs:
     pullImage: true
 
   # techdocs.builder can be either 'local' or 'external.
-  # If builder is set to 'local' and you open a TechDocs page, techdocs-backend will try to generate the docs, publish to storage
-  # and show the generated docs afterwords. This is the "Basic" setup of the TechDocs Architecture.
-  # If builder is set to 'external', techdocs-backend will only fetch the docs and will NOT try to generate and publish. In this case of 'external',
-  # we assume that docs are being built by an external process (e.g. in the CI/CD pipeline of the repository). This is the "Recommended" setup of
-  # the architecture. Read more here https://backstage.io/docs/features/techdocs/architecture
+  # Using the default build strategy, if builder is set to 'local' and you open a TechDocs page,
+  # techdocs-backend will try to generate the docs, publish to storage and show the generated docs afterwords.
+  # This is the "Basic" setup of the TechDocs Architecture.
+  # Using the default build strategy, if builder is set to 'external' (or anything other than 'local'), techdocs-backend
+  # will only fetch the docs and will NOT try to generate and publish.
+  # In this case, we assume that docs are being built by an external process (e.g. in the CI/CD pipeline of the repository).
+  # This is the "Recommended" setup of the architecture.
+  # Note that custom build strategies may alter this behaviour.
+  # Read more about the "Basic" and "Recommended" setups here https://backstage.io/docs/features/techdocs/architecture
+  # Read more about build strategies here: https://backstage.io/docs/features/techdocs/concepts#techdocs-build-strategy
 
   builder: 'local'
 

docs/features/techdocs/how-to-guides.md

@@ -538,3 +538,53 @@ Done! Now you have a support of the following diagrams along with mermaid:
 - `Vega`
 - `Vega-Lite`
 - `WaveDrom`
+
+## How to implement a hybrid build strategy
+
+One limitation of the [Recommended deployment](./architecture#recommended-deployment) is that
+the experience for users requires modifying their CI/CD process to publish
+their TechDocs. For some users, this may be unnecessary, and provides a barrier
+to entry for onboarding users to Backstage. However, a purely local TechDocs
+build restricts TechDocs creators to using the tooling provided in Backstage,
+as well as the plugins and features provided in the Backstage-included `mkdocs`
+installation.
+
+To accommodate both of these use-cases, users can implement a custom [Build Strategy](./concepts#techdocs-build-strategy)
+with logic to encode which TechDocs should be built locally, and which will be
+built externally.
+
+To achieve this hybrid build model:
+
+1. In your Backstage instance's `app-config.yaml`, set `techdocs.builder` to
+   `'local'`. This ensures that Backstage will build docs for users who want the
+   'out-of-the-box' experience.
+2. Configure external storage of TechDocs as normal for a production deployment.
+   This allows Backstage to publish documentation to your storage, as well as
+   allowing other users to publish documentation from their CI/CD pipelines.
+3. Create a custom build strategy, that implements the `DocsBuildStrategy` interface,
+   and which implements your custom logic for determining whether to build docs for
+   a given entity.
+   For example, to only build docs when an entity has the `company.com/techdocs-builder`
+   annotation set to `'local'`:
+   ```typescript
+   export class AnnotationBasedBuildStrategy {
+     private readonly config: Config;
+
+     constructor(config: Config) {
+       this.config = config;
+     }
+
+     async shouldBuild(_: Entity): Promise<boolean> {
+       return this.entity.metadata?.annotations?.["company.com/techdocs-builder"] === 'local'
+     }
+   }
+   ```
+4. Pass an instance of this Build Strategy as the `docsBuildStrategy` parameter of the
+   TechDocs backend `createRouter` method.
+
+Users should now be able to choose to have their documentation built and published by
+the TechDocs backend by adding the `company.com/techdocs-builder` annotation to their
+entity. If the value of this annotation is `'local'`, the TechDocs backend will build
+and publish the documentation for them. If the value of the `company.com/techdocs-builder`
+annotation is anything other than `'local'`, the user is responsible for publishing
+documentation to the appropriate location in the TechDocs external storage.