With the addition of ruby.withPackages, @manveru rewrote the nixpkgs
manual section for the ruby language but did not add it to the manual.
This commit replaces the previous documentation with manveru's updated
version.

Motivation for this change

The updated ruby language section written by @manveru seems to be the most current documentation for using ruby within the Nix ecosystem. Despite being authored almost 10 months ago, it has never been added to the nixpkgs manual visible at nixos.org because it was not added to the index file for the languages and frameworks section. I think it should be.

Changes made

• Added doc/languages-frameworks/ruby.section.md to doc/languages-frameworks/index.xml
• Updated headings to resolve error messages over conflicting section ids

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.