feat: move RCP docs into Welcome section

[dtKinger]

... and update sidebar, and all links to RCP page before redirects needed

This PR condenses sidebar some, improve the information architecture. If we were post-launch we would need to set up URL redirects but at this phase, I can just update the cross-links within the site.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 764af93):

https://knokd-docs-30d6e--pr21-edit-ia-some-og1f91pp.web.app

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