Added llms.txt

[forman]

Added llms.txt.

Closes #1193

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.54%. Comparing base (e39e3f9) to head (1190fbe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1194   +/-   ##
=======================================
  Coverage   89.54%   89.54%           
=======================================
  Files         280      280           
  Lines       21655    21655           
=======================================
  Hits        19391    19391           
  Misses       2264     2264           

☔ 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.