Add docs for email 2FA

[jhutchings1]

Summary

Adds docs for email 2FA (ref: PCX-19798)

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 change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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 PR requires additional review attention because it affects the following areas:

Partials

This PR updates partial files, which are pieces of content used across multiple files in our Render component.

• support/2fa-enable - view affected files

[github-actions]

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

Pattern	Owners
/src/content/docs/fundamentals/	@dcpena, @cloudflare/pcx-technical-writing
*	@cloudflare/pcx-technical-writing

[jhutchings1]

BTW, I wasn't able to do a local build here, so appreciate a sanity check on the links. For whatever reason, I was getting errors when running astro dev.

16:43:28 [ERROR] Cannot find module 'undefinedasides.css' imported from 'virtual:starlight/user-css'
  Stack trace:
    at /Users/jhutchings/apps/cloudflare/cloudflare-docs/node_modules/vite/dist/node/chunks/dep-Bu492Fnd.js:46831:19
    [...] See full stack trace in the browser, or rerun with --verbose.
16:43:28 [ERROR] Cannot find module 'undefinedasides.css' imported from 'virtual:starlight/user-css'
  Stack trace:
    at fetchModule (file:///Users/jhutchings/apps/cloudflare/cloudflare-docs/node_modules/vite/dist/node/chunks/dep-Bu492Fnd.js:46831:19)
    [...] See full stack trace in the browser, or rerun with --verbose.

[github-actions]

Preview URL: https://133aac92.preview.developers.cloudflare.com
Preview Branch URL: https://jhutchings1-email-2fa.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/fundamentals/user-profiles/2fa/	https://jhutchings1-email-2fa.preview.developers.cloudflare.com/fundamentals/user-profiles/2fa/