Replace custom Indexer system with standard gemspec, Travis CI with
GitHub Actions, and Assembly/detroit/ergo with Rakefile. Fix Fixnum
reference for Ruby 3+ compatibility. Simplify version.rb. Fix typos,
update URLs to HTTPS, move site from gh-pages to docs/, and clean up
obsolete files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>