Sync public contributor guide with current internal guidance

[M. Baldwin (msmbaldwin)]

Summary

This PR brings the public contributor guide back in sync with the current authoritative internal guidance, and ports over a few externally-relevant topics that were missing. All changes are factual/procedural — public tone and structure are preserved. Opened as a draft for review.

Factual/procedural fixes to existing articles

Article	Change
how-to-write-links.md	Add FWLinks / aka.ms guidance (don't use ShortLinks in Learn content; FWLink /p/ only as external last resort).
metadata.md	Add ms.topic and ms.service to the required metadata attributes (were missing).
how-to-create-github-issues.md, provide-feedback.md	Update feedback control names to current UI: This page / This product.
process-pull-request.md	Clarify that the merge method is repository-specific (#sign-off vs. Squash and merge), not universal.
create-pull-request.md	Note the release-branch exception for the base branch; fix PR-title default wording.
how-to-write-major-edits.md	Add a note about basing work on a release branch when the repo uses one instead of main.
how-to-write-docs-auth-pack.md	Add Learn Scaffolding and Cleanup scripts to the bundled extensions list.
get-started-setup-tools.md	Link to the new AI-assisted authoring tools article.

New articles (ported from internal guidance, made external-appropriate)

• Contributor License Agreement (contributor-license-agreement.md) — the CLA flow for community contributors, incl. the @microsoft-github-policy-service bot commands.
• Accessibility and alt text (accessibility.md) — meaningful alt text, accessible tables/links, and build validation.
• Add images and screenshots (add-images.md) — image syntax, storage/naming, compression, alt text, and secure screenshots (scrub PII).
• AI-assisted authoring tools (ai-authoring-tools.md) — optional Learn Authoring Assistant and GitHub Copilot, alongside the Learn Authoring Pack.

TOC.yml updated for the new articles; ms.date refreshed on edited files.

Notes for reviewers

• Content was cross-checked against the internal guide; a few originally-suspected divergences were verified as already-correct in the public guide and intentionally left unchanged (e.g., the inline-HTML rule and dev_langs coverage).
• The new articles avoid internal-only tooling and don't require anything a public contributor can't do.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

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[learn-build-service-prod]

Learn Build status updates of commit 81006a8:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
Contribute/content/accessibility.md	⚠️Warning	View	Details
Contribute/content/add-images.md	⚠️Warning	View	Details
Contribute/content/ai-authoring-tools.md	⚠️Warning	View	Details
Contribute/content/contributor-license-agreement.md	⚠️Warning	View	Details
Contribute/content/how-to-write-docs-auth-pack.md	💡Suggestion	View	Details
Contribute/content/metadata.md	💡Suggestion	View	Details
Contribute/content/create-pull-request.md	✅Succeeded	View
Contribute/content/get-started-setup-tools.md	✅Succeeded	View
Contribute/content/how-to-create-github-issues.md	✅Succeeded	View
Contribute/content/how-to-write-major-edits.md	✅Succeeded	View
Contribute/content/process-pull-request.md	✅Succeeded	View
Contribute/content/provide-feedback.md	✅Succeeded	View
Contribute/content/TOC.yml	✅Succeeded	View

Contribute/content/accessibility.md

• Line 4, Column 9: [Warning: author-not-found - See documentation] Invalid value for author: 'cahublou' is not a valid GitHub ID.

Contribute/content/add-images.md

• Line 4, Column 9: [Warning: author-not-found - See documentation] Invalid value for author: 'cahublou' is not a valid GitHub ID.

Contribute/content/ai-authoring-tools.md

• Line 7, Column 9: [Warning: author-not-found - See documentation] Invalid value for author: 'cahublou' is not a valid GitHub ID.

Contribute/content/contributor-license-agreement.md

• Line 4, Column 9: [Warning: author-not-found - See documentation] Invalid value for author: 'cahublou' is not a valid GitHub ID.

Contribute/content/how-to-write-docs-auth-pack.md

• Line 8, Column 12: [Suggestion: ms-author-invalid - See documentation] Invalid value for 'ms.author', 'mbradley' is not a valid Microsoft alias.

Contribute/content/metadata.md

• Line 5, Column 12: [Suggestion: ms-author-invalid - See documentation] Invalid value for 'ms.author', 'sabarret' is not a valid Microsoft alias.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.