docs: document solver-specific statistics in solve_problems

[arnavk23]

Added a "Solver-specific statistics" section to the solve_problems docstring in run_solver.jl. The new text explains how entries in s.solver_specific are handled:

• scalar (non-vector) entries become additional DataFrame columns;
• vector-valued entries are ignored for column creation;
• columns are created when the first successful run returns those keys;
• skipped or exception rows are filled with missing for solver-specific columns;
• how to set solver-specific values from a solver (e.g. via callbacks and set_solver_specific!), with tests/examples already exercising the behavior.

Fixes #152

[arnavk23]

@tmigot please review this pr.

[tmigot]

I don't think we need to know all that, only that solver_specific is taken into accounts and columns are created for those.

We should try to be as concise as possible here, and maybe move this next to coltitle.

[arnavk23]

@tmigot I think we can merge this now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.52%. Comparing base (4c0fcd2) to head (2609f5e).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
+ Coverage   76.71%   85.52%   +8.80%     
==========================================
  Files          12       10       -2     
  Lines         292      297       +5     
==========================================
+ Hits          224      254      +30     
+ Misses         68       43      -25     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.