docs: update README skills section with published docs link

[Ian Alton (ian-at-airbyte)]

Summary

Updates the "Claude Code Skills" section of the repo README to:

• Link to the published Claude Code skills documentation on the Airbyte docs site (companion PR: docs(ai-agents): add Claude Code skills documentation page airbyte#74901)
• Link to Anthropic's Claude Code skills documentation for context on what skills are
• Point skill links to README.md instead of SKILL.md (README is the human-readable entry point; SKILL.md is for Claude to consume)
• Add more descriptive text for each skill
• Restructure install instructions with proper markdown headers for readability

Review & Testing Checklist for Human

• Verify the docs link (https://docs.airbyte.com/ai-agents/skills) will resolve once the companion docs PR (docs(ai-agents): add Claude Code skills documentation page airbyte#74901) is merged. These two PRs should be merged in the correct order (airbyte first) or the link will 404 temporarily.
• Confirm the reduced prominence of md2gdoc in install examples is acceptable — it's still listed under "Available skills" but removed from the marketplace and manual install code blocks.

Notes

• This is a documentation-only change to the repo README. No code or skill content is modified.
• The manual install example now only shows airbyte-agent-connectors to keep the example focused. Users can adapt it for md2gdoc by following the same pattern.

Link to Devin session: https://app.devin.ai/sessions/5cd8525606b34e76bb24e36f3c4d50df
Requested by: Ian Alton (@ian-at-airbyte)

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✅ Devin Review: No Issues Found

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