Document web-admins team w/ minimal onboarding

[MattIPv4]

Using this to document my onboarding following #88:

• Add the Nominee to the @nodejs/web-admins team on GitHub.
• Add the Nominee to the @nodejs/web team on GitHub as a maintainer.
• Grant the Nominee Org Admin access to the Node.js Status Page.
• Grant the Nominee Owner access to the Vercel account for Node.js.
• Grant the Nominee Owner access to the Sentry account for Node.js.
• The Nominee should open an issue in nodejs/build requesting admin access to Cloudflare.

[Copilot]

Pull request overview

Updates onboarding and repo documentation to support minimal onboarding for the @nodejs/web-admins team and improve consistency by linking referenced GitHub teams.

Changes:

• Add a new onboarding checklist for @nodejs/web-admins.
• Convert plain-text team mentions in onboarding docs/README into links to the corresponding GitHub teams.
• Add a @nodejs/web-admins section to MEMBERS.md and update README’s sub-team description.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
onboarding/web-infra.md	Links @nodejs/web-infra team in the onboarding checklist.
onboarding/web-admins.md	New onboarding checklist for the Web Admins team.
onboarding/nodejs-website.md	Links @nodejs/nodejs-website team in the onboarding checklist.
README.md	Links @nodejs/web and updates sub-team structure text + mentions Web Admins.
MEMBERS.md	Adds a new Web Admins team membership section.

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