docs: Update README with supported languages for EPUB (#530)

## Summary

- Update README with the list of supported languages for EPUB files.
- Update USER_GUIDE with an extended list of supported and unsupported
languages.

## Additional Context

For weeks, I thought this firmware only supported English, because I
remember you saying that full language support would only be possible
after implementing proper font rendering. I also remember mentioning a
separate Korean fork, Vietnamese issues and so on.

All of this made it clear that this system doesn't support my languages.
I was surprised when I saw a Reddit post with a photo of a book in my
native language. Only then I did learn that such languages ​​are
supported. Therefore, mentioning the supported languages ​​would help
future buyers and new users.

---

### AI Usage

Did you use AI tools to help write this code? _**NO**_

README.md

@@ -41,7 +41,9 @@ This project is **not affiliated with Xteink**; it's built as a community projec
   - [ ] Full UTF support
 - [x] Screen rotation
 
-See [the user guide](./USER_GUIDE.md) for instructions on operating CrossPoint.
+Multi-language support: Read EPUBs in various languages, including English, Spanish, French, German, Italian, Portuguese, Russian, Ukrainian, Polish, Swedish, Norwegian, [and more](./USER_GUIDE.md#supported-languages).
+
+See [the user guide](./USER_GUIDE.md) for instructions on operating CrossPoint. 
 
 ## Installing