Improve usage overview docs

[bwohlberg]

• Description of inverse problems in Overview section no longer ADMM-specific.
• Add overview on optimizers to Important Classes section.

[codecov]

Codecov Report

Merging #76 (9d8453d) into main (ff97e6d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #76   +/-   ##
=======================================
  Coverage   89.12%   89.12%           
=======================================
  Files          43       43           
  Lines        2978     2978           
=======================================
  Hits         2654     2654           
  Misses        324      324           

Flag	Coverage Δ
unittests	89.12% <ø> (ø)

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