Refactor README.md

[jack-berg]

Now that I've finished refactoring opentelemetry.io Java docs, I figured its time to give the project README a face lift.

The current README tries to perform some of the functions which are now done at a higher level in opentelemetry.io. The result is cluttered and out of date.

Ideas in this PR:

• The readme should more or less be a routing page, with just enough details on various key subjects to connect with a reader and link to a more exhaustive document
• Start with terse explanation of repository, quickly redirecting users to the opentelemetry.io source of truth for more details
• Quick introduction of things like requirements, releases, compatibility, contributing, but link out to more detailed explanations in VERSIONING.md, CONTRIBUTING.md, RELEASING.md, etc.
• Move tables summarizing artifacts to collapsible sections, with API and SDK sections expanded by default
• Move various gradle / maven instructions to collapsible sections, with gradle instructions expanded by default

Result is README reduces from ~8200 pixels of content to ~4400 (~46%).

Hopefully this reduction makes it easier for users to consume the content and reach the information they need faster.

To compare and contrast, see current and newly proposed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.30%. Comparing base (ca98fc5) to head (b578638).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6881   +/-   ##
=========================================
  Coverage     90.30%   90.30%           
  Complexity     6589     6589           
=========================================
  Files           729      729           
  Lines         19767    19767           
  Branches       1944     1944           
=========================================
  Hits          17851    17851           
  Misses         1325     1325           
  Partials        591      591           

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[jaydeluca]

looks nice