Open a draft documentation PR when code merges to main

[dhruv8sh]

Summary

• When code merges to main, open a draft documentation PR so the doc updates for that change have a home and don't get forgotten.
• The draft PR branches from the merged commit (docs/<sha>) with one empty commit, links the source PR, and lists the files the merge changed.
• Merges that only touch docs/** or Markdown are skipped.

Changes

File	Change
.github/workflows/docs-pr.yml	New workflow on push to main (ignoring docs/Markdown-only pushes); job-scoped contents: write and pull-requests: write.
scripts/open-docs-pr.sh	Creates the docs/<sha> branch, pushes it, and opens the draft PR with gh. Falls back to the commit's own diff when the push's before-SHA is all zeros. Caps the file list at 100. DRY_RUN=1 prints the PR instead.
scripts/README.md	Adds the script to the scripts table (Prettier re-padded the existing rows).

Closes

Closes #1105

Test plan

• cargo test-fastly && cargo test-axum (plus cargo test-cloudflare)
• cargo clippy-fastly && cargo clippy-axum (plus cargo clippy-cloudflare, cargo check-cloudflare)
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run (on the pinned Node 24.12.0)
• JS format: cd crates/trusted-server-js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other:
  • actionlint on the workflow and shellcheck on the script: clean.
  • DRY_RUN=1 against Make body hold parser-aware and stream Next.js processing #1135's merge commit: correct title, source PR resolved to Make body hold parser-aware and stream Next.js processing #1135, all 29 changed files listed; the all-zero before-SHA fallback produces the same result.
  • Full path in a scratch clone pushing to a local bare remote with a stub gh: the branch has exactly one empty commit on the merge commit, and gh pr create --draft --base main receives the expected title and body.
  • The workflow itself was not run end to end (that would push to this repository).

Notes for reviewers

• The repository setting Allow GitHub Actions to create and approve pull requests must be enabled for the workflow's gh pr create to succeed.
• PRs opened with the default GITHUB_TOKEN do not trigger other workflows, so CI will not run on these draft PRs until someone pushes to them. Swapping in a GitHub App token would change that if CI on open is wanted.

Checklist

• Changes follow AGENTS.md conventions
• No unwrap() in production code — use expect("should ...") (no Rust changes)
• Uses tracing macros (not println!) (no Rust changes)
• New code has tests (shell/workflow only; verified with the dry-run and local-remote runs above)
• No secrets or credentials committed