Document the new export formats

https://redmine.teklia.com/issues/7497

The new PDF and PAGE XML exports introduced in frontend#1507 (closed) should be documented.

The How to → Export a project is now a section. The current page becomes a sub-page for SQLite exports.

A new page for PDF and PAGE XML exports should be added. I'm not sure if one page per format is really needed since the entire process is the same, unless we want to copy the documentation of the optional parameters of each worker. That page includes how to start an export and how to download it through the status page.

The Projects → Exports page should be renamed Projects → SQLite exports, to avoid any confusion.