Document corpus export

• add an "how to export a Project", describring with screenshot the workflow:
  • where is the button to list & start export
  • deletion after 2 weeks
  • email sent
• mention the project is exportable in the project intro page, with a link toward the how to export
• add a new page describing the export database in Projects > Export
  • make a graph with mermaid (at the table level, not fields)
  • convert that note into a table:
    • each table's field has a line, with a description and type
  • add a section with a few sample queries:
    • list all elements with type page
    • list page elements from a given folder (using CTE)
    • list all transcriptions for the lines under a page (no CTE, direct join on one level)