RDoc-3882 Document GitHub and Google OAuth login support

[PFYasu]

Issue link

https://issues.hibernatingrhinos.com/issue/RDoc-3882

Additional description

Documents the new GitHub and Google OAuth sign-in flow in RavenDB Cloud (RDBCL-5177 / Cloud PR #4215).

cloud/cloud-overview.mdx

• Section ## Login To Your Account — refreshed intro noting that users can sign in either with a magic link or via GitHub/Google.
  • Existing magic-link content kept under a new #### Login with a magic link heading.
  • New #### Login with GitHub or Google subsection covering:
    • The two new Login via GitHub / Login via Google buttons on the login page
    • Flow: authorize at the provider, return to RavenDB Cloud, match by email
    • 2FA behavior (prompted after OAuth authorization, same as for magic link)
    • "No Cloud account for the email" path: a Check your inbox page is shown and a sign-in / registration link is emailed, with the registration form pre-filled with the email
• Section ## Register Your Account — short info admonition noting that registration can also be initiated from the login page via GitHub or Google.
• In-page TOC updated with the new login subsections.

Screenshots

Two screenshots added:

• cloud/assets/login-page-oauth-buttons.png — login page showing GitHub / Google buttons
• cloud/assets/login-oauth-check-email-page.png — Check your inbox page after OAuth login with an unregistered email

Type of change

• Content - docs
• Content - cloud
• Content - guides
• Content - start pages/other
• New docs feature (consider updating /templates or readme)
• Bug fix
• Optimization
• Other

Changes in docs URLs

• No changes in docs URLs
• Articles are restructured, URLs will change, mapping is required (update /scripts/redirects.json file, set Documents Moved PR label)

Changes in UX/UI

• No changes in UX/UI
• Changes in UX/UI (include screenshots and description)