doc: add large pull requests contributing guide

[mcollina]

Fixes: #62752

Adds a contributing guide for large pull requests (3000+ lines), covering:

• Threshold definition and applicability
• Restriction to collaborators only
• Design document and review guide requirements
• Two TSC approvals (same as semver-major)
• Dependency change separation
• Feature fork workflow in separate repositories
• Reviewer guidance

[nodejs-github-bot]

Review requested:

• @nodejs/tsc

[panva]

• We should link to it from CONTRIBUTING.md, pull-requests.md, collaborator-guide.md (or some subset of those)
• We should exclude routine dependency/wpt/test bot-issued PRs, this can either be excluded explicitly with additional rules or by changing the classification put forth by the Overview or What qualifies as a large pull request sections
• dependency changes in a separate commit is at odds with having each commit self-contained, this is fine when the commits get squashed but the first commit's message is used for the squash so the first commit would be the non-deps changes? seems odd and counter-intuitive
• Design document - I 100% discourage linking outside documents, a detailed PR description should also suffice, 3000 can easily be hit with just thorough test coverage for an otherwise trivial change, how would a design document help such PRs?
• Both Separating test additions from implementation changes. and Splitting documentation into its own pull request. strategies to avoiding large PRs are contrary to what we usually request of contributions. Those strategies only work when the contribution isn't "done" or "shipped" and is excluded from builds.