doc: move solvers doc to src\solvers.md

[inkydragon]

When building julia with USE_GPL_LIBS:=0, the documentation builds fails because the documentation for the solver cannot be found.

Only docs/src/index.md is copied when building the documentation, so we can move the solver documentation to docs/src/solver.md to avoid this problem.

See: JuliaLang/julia#56344

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.00%. Comparing base (485fd4b) to head (9bade12).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
- Coverage   84.14%   84.00%   -0.15%     
==========================================
  Files          12       12              
  Lines        9160     9160              
==========================================
- Hits         7708     7695      -13     
- Misses       1452     1465      +13     

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[ViralBShah]

@dkarrasch It would be good to pull this into the 1.10 and 1.11 PRs you already have open.