Add LLM friendly documentation (llms.txt)

[csvance]

Since there are no public examples of using gRPCClient.jl v1.0.0 to my knowledge in real projects, its unlikely to be included as data for LLM pretraining. In order to facilitate usage of this project by LLM coding assistants, provide LLMS.md, essentially just a markdown version of the documentation produced by Documenter.jl. Since Documenter.jl does not currently support Markdown output, I forked it and used LLM assistance in order to add it just for the purposes of being able to generate LLMS.md rather than to upstream it.

[csvance]

• Add llms.txt to docs/src so its included in static documentation website.
• Also reference it in README.md

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.58%. Comparing base (7cfad6b) to head (ce34a3d).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #103   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files           7        7           
  Lines         531      531           
=======================================
  Hits          481      481           
  Misses         50       50           

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