TechDocs: Update docs around docker-in-docker installation

Fix python package name to mkdocs-techdocs-core
Refer to the original Dockerfile where users can seek inspiration from

docs/features/techdocs/getting-started.md

@@ -73,10 +73,10 @@ required.
 
 ### Disable Docker in Docker situation (Optional)
 
-The TechDocs backend plugin runs a docker container with mkdocs to generate the
-frontend of the docs from source files (Markdown). If you are deploying
-Backstage using Docker, this will mean that your Backstage Docker container will
-try to run another Docker container for TechDocs backend.
+The TechDocs backend plugin runs a docker container with mkdocs installed to
+generate the frontend of the docs from source files (Markdown). If you are
+deploying Backstage using Docker, this will mean that your Backstage Docker
+container will try to run another Docker container for TechDocs backend.
 
 To avoid this problem, we have a configuration available. You can set a value in
 your `app-config.yaml` that tells the techdocs generator if it should run the
@@ -90,10 +90,16 @@ techdocs:
 ```
 
 Setting `generators.techdocs` to `local` means you will have to make sure your
-environment is compatible with techdocs. You will have to install the
-`mkdocs-techdocs-container` and 'mkdocs' package from pip, as well as graphviz
-and plantuml from your package manager. This has only been tested with python
-3.7 and python 3.8.
+environment is compatible with techdocs.
+
+You will have to install the `mkdocs-techdocs-core` and `mkdocs` package from
+pip, as well as `graphviz` and `plantuml` from your OS package manager (e.g.
+apt). See our
+[Dockerfile](https://github.com/spotify/backstage/blob/master/packages/techdocs-container/Dockerfile)
+for the latest requirements. You should be trying to match your Dockerfile with
+this one.
+
+It is worth mentioning that we recommend Python version 3.7 or higher.
 
 ## Run Backstage locally