Add Sign in with GitHub docs and changelog

[jhutchings1]

Summary

Adds a changelog and docs for sign in with GitHub.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/changelog/	@cloudflare/pm-changelogs, @cloudflare/pcx-technical-writing
/src/content/docs/fundamentals/	@dcpena, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://50cb86f5.preview.developers.cloudflare.com
Preview Branch URL: https://sign-in-with-github.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/fundamentals/user-profiles/login/	https://sign-in-with-github.preview.developers.cloudflare.com/fundamentals/user-profiles/login/
https://developers.cloudflare.com/changelog/2025-09-25-sign-in-with-github/	https://sign-in-with-github.preview.developers.cloudflare.com/changelog/2025-09-25-sign-in-with-github/