docs: update documentation for repogen

[frostyardyeti]

Summary

Updates the project overview documentation to reflect recent code changes, including the Homebrew bottle parsing behavior, pre-compiled regex optimization, and GPG key URL variable support. These additions keep the documentation in sync with the current codebase.

Changes

• Clarified that Homebrew bottles skip parsing and derive metadata from the filename, unlike other package formats
• Added a new "Pre-compiled Regexes" section documenting the package-init-time regex compilation optimization for RPM and Homebrew parsing loops
• Updated the --gpg-key-url flag description to note support for $releasever/$basearch variables