Add support for generation of varcond documentation

[wadoon]

Intended Change

Let class de.uka.ilkd.key.nparser.varexp.TacletBuilderManipulators#main()
print out Markdown documentation for the current known varconds.

Added therapi-javadoc to write Javadoc into resources to be read at runtime.

Type of pull request

• New feature (non-breaking change which adds functionality)

Ensuring quality

• I made sure that introduced/changed code is well documented (javadoc and inline comments).
• I made sure that new/changed end-user features are well documented (https://github.com/KeYProject/key-docs).
• I added new test case(s) for new functionality.
• I have tested the feature as follows: ...
• I have checked that runtime performance has not deteriorated.
• For new Gradle modules: I added the Gradle module to the test matrix in
  .github/workflows/tests.yml

[wadoon]

We might want to discuss whether we want to introduce this library https://github.com/dnault/therapi-runtime-javadoc

Which brings annotations and an annotation processor that writes annotations capturing the Javadoc to Clazz and other Java entities for runtime usage