tcld docs gen

[prasek]

What was changed

• Added tcld docs gen for Nexus commands
• Added docs gen for other tcld commands (Experimental)
  • some sync with published docs is needed to align the two

See related

• tcld nexus docs documentation#3151
• Update nexus commands.yml for docs gen cli#693

Why?

Needed to create nexus.mdx and wanted to use docs gen

Checklist

1. How was this tested:

• go run ./app/internal/cmd/gen-docs
• copied generated nexus.mdx to temporalio/documentation
• yarn start
• verified via http://localhost:3000/

2. Any docs updates needed?

• tcld nexus docs documentation#3151

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