Descriptive error page on MISSING_MATRIX_RTC_FOCUS

### Your use case

#### What would you like to do?

When an app encounters the `MISSING_MATRIX_RTC_FOCUS`, it should display a friendly explanatory page that informs the user that MatrixRTC calling is not set up for their server, and that they should refer to their server's documentation

#### Why would you like to do it?

The current situation is often slightly confusing to users, resulting in reports of "calls don't work" and similar. This is seen even in cases like the recent Gamer's Nexus video. It would help improve reputation and decrease support burden.

### Have you considered any alternatives?

Alternatively, the calling implementation could have a functional fallback mode when the SFU is missing. This is likely blocked on implementing full mesh and/or cost concerns for a fallback mode, though.

### Additional context

_No response_

Start by locating where MISSING_MATRIX_RTC_FOCUS is surfaced in the Element client and review the MatrixRTC and SFU context in this issue. The work is done when users see a friendly page explaining that MatrixRTC calling is not configured for their server and directing them to the server's documentation.

Written by the indexing model from the issue text.