Update user guide for changes brought about by this epic: <mention class="mention" data-id="41351" data-type="work_package" data-text="#41351">#41351</mention>. This includes adding documentation concerning:

* Upload files from file system to Nexctcloud and link to work package
* Link existing file on NC to a work package
* Breadcrumbs to navigate
* Remove files links Begin of the insertion

Included subtasks:

* [x] Write first draft of documentation (text)
* [ ] Review by <mention class="mention" data-id="72512" data-type="user" data-text="@Marc Alcobé">@Marc Alcobé</mention>
* [ ] Link review by Andrej
* [ ] New PR to include images once features are implemented/tested
* [ ] Final review
* [ ] MergeEnd of the insertion