Move docs from readme to docs.rs

[cijothomas]

Continuing from #2336.
Part of #1306

Removes docs from the github readme.md file and moves everything into the docs.rs.
Readme.md is what gets rendered in crates.io, so I'd like to get some feedback from others on how much info we need to put there vs docs.rs...
We want to avoid duplicating information as much as possible.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.5%. Comparing base (9a85ce1) to head (a01fd67).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2341   +/-   ##
=====================================
  Coverage   79.5%   79.5%           
=====================================
  Files        123     123           
  Lines      21365   21365           
=====================================
  Hits       17004   17004           
  Misses      4361    4361           

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[hdost]

I feel like at least for the main opentelemetry it is good to have a more comprehensive page. Giving a good intro before needing to drop into the docs directly.