docs: Replace gh pages with redirects to otel.io registry

[thompson-tomo]

Closes #1435

Instructions on merge steps based on testing done in my fork:

• Update github pages deployment setting to github actions (see screenshow below)
• Merge this pr
• Verify gh pages deployment occured
• Delete gh-pages branch

[chalin]

Thanks @thompson-tomo. I tested the deployment on your fork and it works as well as GitHub Pages allows: browsers get redirected to the registry from any old URL, and the workflow is tidy (pinned SHAs, minimal permissions, persist-credentials: false).

One optional, non-blocking idea: per-gem redirects. All old URLs currently land on the generic registry page, where readers must re-find their gem. Two cheap upgrades:

• Check in a static stub per gem at docs/github_pages/GEM_NAME/latest/index.html redirecting to https://rubydoc.info/gems/GEM_NAME. Being real files, these are served with a 200 status, and readers land on their gem's actual API docs.

• In 404.html, parse the path to route deep links per gem:

  const m = location.pathname.match(/^\/opentelemetry-ruby\/([\w-]+)\//);
  location.replace(m ? `https://rubydoc.info/gems/${m[1]}`
                     : 'https://opentelemetry.io/ecosystem/registry/?language=ruby');

Either way, (RS)LGTM.

[thompson-tomo]

Thanks @chalin, i have made the changes to the 404 page as suggested. I have not bothered with a static per gem page as most visitors will also be passing a version string & often also additional path arguments as that is what was previously used. Hence the static page wouldn't be hit.

Thanks again