docs: replace howto-*.txt with HOWTO-*.md and add AGENTS.md

docs: replace howto-*.txt with HOWTO-*.md and add AGENTS.md

Replace the eight Documentation/howto-*.txt files with four consolidated
markdown documents:

  HOWTO-BUILDING.md     - compilation and build system internals
  HOWTO-CONTRIBUTING.md - contribution guidelines and PR workflow
  HOWTO-HACKING.md      - usage functions, man pages, debugging
  HOWTO-TESTING.md      - test framework, env variables, fuzz targets

Add AGENTS.md to the project root with AI-specific coding guidance
(code style pitfalls, return conventions, memory management, commit
message format) and links to the new documentation files.

Rename 00-about-docs.txt to 00-about-docs.md with updated references.

Signed-off-by: Karel Zak <kzak@redhat.com>