docs(msgraph): add Azure app registration walkthrough + env var reference

Foundation docs shipped alongside the Graph auth/client code so users
have a working path from zero to a verified token from the moment this
PR lands.

- website/docs/guides/microsoft-graph-app-registration.md: new page
  walking through app registration, client secret, the exact minimum
  Graph API permissions per pipeline capability (transcript-first,
  recording fallback, Graph-mode delivery), admin consent, optional
  Application Access Policy for tenant-scoping, token-flow smoke test
  with the shipped MicrosoftGraphTokenProvider, and a troubleshooting
  table for common AADSTS errors. Includes secret-rotation procedure.

- website/docs/reference/environment-variables.md: new Microsoft Graph
  subsection in Messaging documenting MSGRAPH_TENANT_ID, MSGRAPH_CLIENT_ID,
  MSGRAPH_CLIENT_SECRET, MSGRAPH_SCOPE (default .default),
  MSGRAPH_AUTHORITY_URL (with sovereign-cloud override note for GCC
  High etc.).

- website/sidebars.ts: wire the guide into Guides Tutorials.

The guide pages that cover the webhook listener, pipeline runtime,
operator CLI, and outbound delivery land with their matching PRs. This
one is the standalone prereq that's safe to verify in advance.

Verified via npm run build: no new warnings or errors; page routes
correctly at /docs/guides/microsoft-graph-app-registration.

website/sidebars.ts

@@ -181,6 +181,7 @@ const sidebars: SidebarsConfig = {
         'guides/migrate-from-openclaw',
         'guides/aws-bedrock',
         'guides/azure-foundry',
+        'guides/microsoft-graph-app-registration',
       ],
     },
     {