Custom header and footer for printing (Export Update)

[virgile-dev]

@rl-83 could you add the specs and design that we discussed on this issue ?

[rl-83]

Feature description

Before each download, users can preview the document and:

• Edit the layout using 6 clickable blocks located at the top and bottom of the page. For each block, users can choose between leaving it empty, adding text, including the page number, or inserting an image.
• Import a different layout or save the current one for later. The saved layout is linked only to the user.*
• If they are admins, share the current layout as the default layout for the document.

*To simplify document template management, I have preferred a personal approach: each user manages their own library of templates. This avoids the complexity of multi-level management (organization, group, document). Users can also create templates from any document whose layout has been shared.

Reference prototype on Figma (FR)

[virgile-dev]

Robin added a justify option to the feature https://github.com/suitenumerique/docs/issues/395