Document allowed metadata
https://redmine.teklia.com/issues/11188
Under the Projects section, a new Allowed metadata page should be added. It should include:
- what allowed metadata do, and why they exist;
 - the fact that when there are none, users can't create or update metadata without them;
 - how to create, update or delete them through the frontend, with a screenshot in the project management page;
 - the automatic creation when an instance administrator (not project administrator) or a Ponos task creates metadata;
 - the fact that they are ignored with the free edit mode, a link to the relevant section of the metadata page.
 
The Web interface section of the project page should mention that allowed metadata can also be edited within the project, with a link to the new page.
The Web interface section of the metadata page should have its screenshots updated, and should explain the "Free edit" mode. It should mention that when no allowed metadata exist for the project, the form will not allow creating metadata without the free edit mode, with a link to the allowed metadata page.
The project permissions table should include a line for managing allowed metadata and a line for metadata with free edit.