Documentation restructure and UI guide

[avglassman]

With Robin, I've reorganized the documentation, and we've identified some pages that need to be written.

New here:

• Structure: Documentation now has top-bar sections. The more technical (installation, config, etc) is in the Administration guide, and less technical info (UI maneuvers, for now) are in the User guide. The pages have been reorganized/regrouped and lightly edited for clarity.
• The User guide: It's new and quite simple, definitely room for improvement in the future.

I created several pages for Robin and myself to write/co-write when there's time, and once those are ready, this can be the future structure: Proposed documentation nav tree.md

I suspect we may reorganize a bit yet again (mainly to more effectively group information about credentials, tenants, resources, roles etc that is currently present in multiple categories), but for now, I believe that this will be easier to work with. 🙂

[byewokko]

an idea: we could group the basic admin UI pages such as managing tenants, roles and resources under a new section called "Access control". i 'd say access control is all that most admins want from seacat auth.

[avglassman]

Updated - Now there is an Administration guide section and an Account help section (new documents added there, as we discussed). Aleš already approved, @byewokko can we merge and see?