Document build profiles and properties in docs/developer/BUILD.md

Document build profiles and properties in docs/developer/BUILD.md

In doing so, I also noticed a few things in need of improvement. So,
documenting the build also drive those enhancements, such as
  - the new 'fast-build' profile skipping test compilation and execution
    as well as documentation generation,
  - an option to skip generating source assemblies,
  - to skip unzipping source assemblies if javadoc generation for them
    is to be skipped too,
  - activating the 'create-docs' profile by property which is
    true by default instead of using 'activeByDefault=true', because the
    latter does not work reliably if other profiles are activated
    manually according to a Maven bug that was closed as "won't fix",
  - no longer generating separate javadocs for the 'runtime' module,
    because that module is not deployed and the main artifacts recreate
    Javadocs from scratch for all of their constituent sources anyway.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>