docs: add privacy policy hosting checklist

[jjoonleo]

Advances #435
References #464

Summary

• Adds a privacy policy hosting checklist and evidence form for the public HTTPS URL required by [Release] Host the privacy policy on a public HTTPS URL #435.
• Links the checklist from the docs index and release checklist.
• Keeps the final URL as TBD because [Release] Draft and approve the privacy policy #434 is still open and publishing requires human hosting access.

Verification

• git diff --check
• git diff --cached --check
• rg -n "Privacy Policy Hosting|Final privacy policy URL|curl -I|#436|#437|#441|issue 435" docs/Privacy-Policy-Hosting.md docs/Home.md docs/Release-Checklist.md plans/issue-435-privacy-policy-hosting.md

Remaining human tasks

• Complete [Release] Draft and approve the privacy policy #434 with product/legal-approved privacy policy text.
• Choose or confirm the public hosting surface.
• Publish the approved policy at a public HTTPS URL that is not a PDF, not publicly editable, not geofenced, and works without login.
• Fill in the evidence form in docs/Privacy-Policy-Hosting.md with the final URL and validation results.
• Comment the final URL on [Release] Host the privacy policy on a public HTTPS URL #435, then use the same URL for [Release] Add an in-app privacy policy link #436 and [Release] Enter privacy policy URL in Play Console #437.