Add a solver capability matrix to the docs

[dionhaefner]

Per-solver exclusions (which solver/experiment combinations don't work, and why) are currently scattered across mosaic/benchmarks/problems/*/exclusions.py files. This information is valuable for users choosing solvers but hard to discover.

Goal: Generate a human-readable capability matrix and include it in the docs. For each domain, show a table like:

Solver	forward	gradient	cost	optimization	Notes
FEniCS	✓	✓	✓	✓
deal.II	✓	—	fwd only	—	No AD path (C++)
...

This could be:

• A static table in docs/solvers.qmd, or
• Auto-generated from the exclusion data (similar to how docs/generate.py builds the solver reference)

The exclusion objects already carry structured category and reason fields, so the data is machine-readable.