docs: document Band and BandError

[ChrisRackauckas-Claude]

Summary

Documents Band and BandError at their source definitions, including fields, arguments, and executable examples. Adds both names to the existing rendered API page.

This is the upstream dependency for FastAlmostBandedMatrices.jl removing its local public-doc QA suppression.

Ignore until reviewed by @ChrisRackauckas.

Verification

• julia --project -e 'using Pkg; Pkg.test()'\n - Overall | 30727 passed, 5 broken, 30732 total\n- julia --project=docs docs/make.jl\n - Doctest, document checks, and HTML rendering completed. The existing build emitted six :missing_docs warnings under its pre-existing warnonly configuration.\n- typos --format brief src/generic/Band.jl docs/src/index.md\n- git diff --check\n\n🤖 Generated with Claude Code\nhttps://chatgpt.com/codex/tasks/019f4028-d2ae-7a12-aff0-7d996bd9c791

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (f5403f4) to head (15ca6e0).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #507       +/-   ##
===========================================
+ Coverage    0.00%   90.13%   +90.13%     
===========================================
  Files          25       27        +2     
  Lines        3714     3740       +26     
===========================================
+ Hits            0     3371     +3371     
+ Misses       3714      369     -3345     

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