Migrate Ruby docs to this repo

[chalin]

Contributes to #730. This is the otel.io portion of the work to be done for:

• Move website_docs back to website repo opentelemetry-ruby#1395

Tasks:

• Update otel-ruby submodule on website
• Copy website-docs content from submodule into content/en/docs/instrumentation/ruby (requires replacing the symlink with a real folder)
• Drop the otel-ruby submodule
• Ensure that building the site generates the same site files
• Adjust github_repo-related front-matter so that links into GH work once again
• CODEOWNERS: grant @open-telemetry/ruby-approvers and @open-telemetry/ruby-contrib-approvers access to the ruby docs folder - Add ruby docs approvers to relevant section in CODEOWNERS #1966