Document the CSES integration in the preferences / extensions docs

[bobeor]

The upcoming CSES integration for CP Editor (cpeditor/cpeditor#1522) adds
a "Submit to CSES" button that appears on autodetected cses.fi problems.
Documentation is needed for the new user-facing settings added to the
Preferences window:

• CSES/Enable — "Enable CSES Integration"
• CSES/Show Toast Messages — "Show toast messages for submission verdicts"
  Please cover:

1. Enabling/disabling CSES integration.
2. The "Submit to CSES" button: how it appears on CSES problems, the
   login flow (opens the auth URL in the browser), and how saved
   credentials/token are stored.
3. The toast messages for submission verdicts (AC / WA etc.).
4. Any CSES-specific caveats (e.g. login expiry, re-login behavior).
   This tracks the docs work for the feature; docs PR to follow before
   the feature PR is merged.