[Doc]: Add `Schema.to_<backend>` methods in API docs

### What type of report is this?

Correction

### Please describe the issue.

#1924 introduced `to_<backend>` methods in `Schema` class, yet these are not reflected in the docs.

### If you have a suggestion on how it should be, add it below.

Add these methods in https://github.com/narwhals-dev/narwhals/blob/main/docs/api-reference/schema.md