Document migration for column-name selection callbacks

[quinnj]

The 1.0 migration guide removes function-valued column selection but does not show how to preserve a predicate that transforms column names. Add an executable example using CSV.lazy, Tables.columnnames, and CSV.File(indexed; drop=...), including case folding and accent removal.

The example preserves original output names and reuses the structural index. It also states the retained-memory cost and where source versus eager parsing options belong. No runtime API changes.

Addresses #1207.

Validation: strict local Documenter build, including the new example and its output assertion; 331,546 package assertions on Julia 1.12 and 331,100 on Julia 1.10; all 11 package-quality checks. All passed.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.75%. Comparing base (c77e39b) to head (0d3d9fe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1208   +/-   ##
=======================================
  Coverage   95.75%   95.75%           
=======================================
  Files           9        9           
  Lines        5489     5489           
=======================================
  Hits         5256     5256           
  Misses        233      233           

☔ View full report in Codecov by Harness.
📢 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.