TeachBooks Package Update

[rlanzafame]

• changed from external/readme to internal features/teachbooks.md https://teachbooks.io/manual/19-update-teachbooks-page-in-manual-replace-readme-submodule/features/teachbooks.html
• added page explaining need and examples of local servers (of which teachbooks serve is 1) https://teachbooks.io/manual/19-update-teachbooks-page-in-manual-replace-readme-submodule/features/local_server.html
• pointed out with user type box on top of page that the info is relevant for all (most??), but implemention not all user types
• updated external/deploy-book-workflow with a ref tag and updated commit (i think!)

[Tom-van-Woudenberg]

• I deleted the submodule
• deploy-book-workflow seems fine!
• I've made the title more descriptive
• Fixed typo

I like the idea to split the teachbook functionaliteit to different features in the manual. I think for the typical manual-reader, it's not clear what But maybe we should do that properly:

• Add a separate page for draft-release workflow (although that workflow is a bit outdated with many branches and proper use of one branch per 'feature')
• Move teachbook local serve to the local_serve page fully.
• Keep the teachbooks page only with a description of the package itself and the 'Jupyter Book Pre- and Post-Processing' as it's describing how other (and future) functionalities work.
  What do you think @rlanzafame?