[docs] rewrite documentation and remove examples

[odow]

Closes #199

I still need to document the quadratic stuff, but maybe that can wait until after #206

https://jump.dev/ParametricOptInterface.jl/previews/PR207/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.19%. Comparing base (ea24c19) to head (5845215).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #207   +/-   ##
=======================================
  Coverage   99.19%   99.19%           
=======================================
  Files           5        5           
  Lines        1487     1487           
=======================================
  Hits         1475     1475           
  Misses         12       12           

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