docs: release 2.7 and user guide rewrite
Closes #1030 (closed) and introduces a new structure for the user guide so that it is organised into sections according to user goals:
- Getting Started with Ketty
- Managing Books and Collaboration
- Content Creation and Writing
- Book Design and Layout
- Preview and Publish
- Administration
This is basically a rewrite of the current guide to present information in an easy-to-follow list structure. Images have also been limited to the "Understanding the Ketty Interface" page to avoid frequent updates.
Edited by Dione Mentis