Add techdocs-core plugin to techdocs builder automatically

Currently users need to modify their mkdocs files to container techdocs-core plugin manually when they want to expose techdocs in Backstage. To have a standardized set of extensions without the need to modify existing files (and possibly pollute alternative/additional mkdocs pipelines) this should be added automatically.

This PR adds a helper function to modify the mkdocs file to contain this plugin if it doesn't exist in the config file.

Co-authored-by: @emmaindal

Signed-off-by: Jussi Hallila <jussi@hallila.com>

docs/features/techdocs/cli.md

@@ -130,6 +130,8 @@ Options:
                                   if not found.
   --etag <ETAG>                   A unique identifier for the prepared tree e.g. commit SHA. If provided it will be stored
                                   in techdocs_metadata.json.
+  --omitTechdocsCoreMkdocsPlugin  An option to disable automatic addition of techdocs-core plugin to the mkdocs.yaml files.
+                                  Defaults to false, which means that the techdocs-core plugin is always added to the mkdocs file.
   -v --verbose                    Enable verbose output. (default: false)
   -h, --help                      display help for command
 ```

docs/features/techdocs/configuration.md

@@ -37,6 +37,11 @@ techdocs:
 
     pullImage: true
 
+    mkdocs:
+      # (Optional)  techdocs.generator.omitTechdocsCoreMkdocsPlugin can be used to disable automatic addition of techdocs-core plugin to the mkdocs.yaml files.
+      # Defaults to false, which means that the techdocs-core plugin is always added to the mkdocs file.
+      omitTechdocsCorePlugin: false
+
   # 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.

docs/features/techdocs/creating-and-publishing.md

@@ -79,6 +79,9 @@ plugins:
   - techdocs-core
 ```
 
+> Note - The plugins section above is optional. Backstage automatically adds the `techdocs-core` plugin to the
+> mkdocs file if it is missing. This functionality can be turned off with a [configuration option](./configuration.md) in Backstage.
+
 Update your component's entity description by adding the following lines to its
 `catalog-info.yaml` in the root of its repository: