Add links to API docs in readme

[st0012]

While we've added quite extensive API docs to the SDK (at least to sentry-ruby), it's not easy to find them, which could contribute to misuses/confusion of APIs. Therefore, I think we should make the documentation more discoverable by linking them in the gems table.

And because the table already takes quite big of a space, I decided to remove the downloads column to make room for API doc links. I'm also ok with replacing the coverage column instead though.

For this initial improvement, I chose to use rubydoc.info as the source as it automatically fetches the gems' source and generate the content. I think ideally we can host them with GH pages with a automatic publishing workflow, which will give us more control (example: Ruby LSP). I think this is what sentry-rust does as well?

#skip-changelog

[codecov]

Codecov Report

Merging #2300 (285bd15) into master (02538e4) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2300   +/-   ##
=======================================
  Coverage   98.71%   98.71%           
=======================================
  Files         202      202           
  Lines       13282    13282           
=======================================
  Hits        13111    13111           
  Misses        171      171           

Components	Coverage Δ
sentry-ruby	99.09% <ø> (ø)
sentry-rails	97.37% <ø> (ø)
sentry-sidekiq	96.98% <ø> (ø)
sentry-resque	96.76% <ø> (ø)
sentry-delayed_job	98.91% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)