Document memory_sizes OpenJDK configuration

Document the memory_sizes configuration section of config/openjdk.yml.

Add missing documentation of repository_root for both OpenJDK and
Tomcat configuration files.

Improve the repository documentation.

Add an empty memory_sizes section to config/openjdk.yml to make it
easier for users to configure memory sizes.

[#52618651]
Merge 52618651-doc-memory-sizes to master

[Completes #52618651]