Warn about outdated content & misc. housekeeping & slicing/caching docs

[FliegendeWurst]

Rendered

• Warning labels for outdated pages
• Changelog for 2.12.1
• Hidden page (Hidden doc page: Rewrite of the KeYParser #12) added to navigation
• Duplicate HowtoTaclet removed
• Docs for the external DB of Proof Caching (Proof caching: external database key#3265)
• Docs for Chain Shortening (part of Proof caching: use dependency graph to increase hit rate key#3305)

[FliegendeWurst]

Related: #12

[WolframPfeifer]

Thanks for updates and clean-up!

I think we should keep the old documentation for now (How2ExtRecoder), as Recoder is still part of the project and the documentation might still be helpful for understanding it. I do not know if we have it somewhere else.

Maybe it would also make sense to move the caching documentation to a different PR, since I do not want to have it in the docs until it is actually implemented in KeY that way ...

[WolframPfeifer]

What is this file used for?

[FliegendeWurst]

It specifies the dependencies required by mkdocs. Some new dependency management tools for Python can use this file to automatically set up a dev environment.

But it would certainly be a good idea to unify the dependency info. I think there are at least three different places they are specified at the moment..