Docs - Refactor the sign-in guidance

[Saurabh7019]

The current Sign in docs page mixes three things into one long page:

1. the actual sign-in process
2. automatic Entra App Registration creation
3. a full manual Azure Portal walkthrough (permissions, redirect URIs, advanced settings).

This makes it hard for users to tell what's a one-time prerequisite versus the sign-in steps they need to repeat.

This issue is to split the page into two:

1. Entra App Registration (new page) which covers both automatic (one-click) and manual (Azure Portal) app registration setup, update docs sidebar and add this page before sign-in
2. Sign in page which keeps only the actual sign-in flow - add a clear 'prerequisite' callout at the top linking to the app reg page, open the account panel or run the action, choose an existing app registration