docs(CONTRIBUTING.md): expand with contribution channel guidelines

[mcanouil]

Expand .github/CONTRIBUTING.md into a single entry point for contributors, since the routing between issues, discussions, and other repos was only discoverable from the templates.

• Add a Code of Conduct link in the intro.
• Add a "Choosing the right channel" table routing each need to its destination (issue forms, Discussions, and the Visual Editor / RStudio / Positron repos).
• Expand "Opening an issue" with search-first, pre-release check, quarto check output, and the Bug Reports guide.
• Add "Starting a discussion" describing the three Discussion categories.
• Add "AI-assisted contributions", the reciprocal target of the Code of Conduct clause that points to CONTRIBUTING.md.
• Expand "Submitting a pull request" with a pre-PR checklist mirroring the PR template.

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