docs: switch to sphinx_rtd_theme to match other docs

[joshtrichards]

This switches the Desktop Client Manual to the same theme used for all of our other documentation (i.e. Admin, User, Dev):

• sphinx-rtd-theme + optional dark mode
• unifies some misc related elements (directly copied from our other documentation):
  • custom.css
  • up-to-date logos
• adds a requirements.txt for ease of installation of dependencies for documentation developers/contributors (this is how we do things for the other docs already)
  • e.g. pip install -r requirements.txt
• also includes a couple tiny bug fixes:
  • a table issue in the Advanced Usage chapter that broke the inclusion of the [General] section of the Configuration File section at some point in the last release or so
    • i.e. compare current live for 3.13 to current live for 3.11
  • corrects a couple build time heading length mismatch warnings

Notes:

• I think there is an additional step (automated? manual?) in the build->deploy to production for the Desktop docs. That may need some finessing since as best as I can tell there are some other bits added outside the Sphinx build process for the production deploys that are ending up on https://docs.nextcloud.com/desktop/3.13/ (the outdated in terms of design Nextcloud heading/etc). Presumably those are no longer needed after this change (we don't use them for the other docs at least).
• This should also make it easier to transition the docs out of the Desktop repo and over to the main documentation repo at a later date/in a follow-up if needed.

[nextcloud-desktop-bot]

AppImage file: nextcloud-PR-6854-2ab298e63a8334cf4c5c18a8660c7c9192c60470-x86_64.AppImage

To test this change/fix you can simply download above AppImage file and test it.

Please make sure to quit your existing Nextcloud app and backup your data.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[mgallien]

the doc building CI check via drone is failing
can you have a look or tell us if you would need help ?
I fear we currently use an old docker image when compared to server documentation

[mgallien]

@joshtrichards I am sorry for the delay
this is complicated as the build infrastructure for desktop files client is really outdated compared to the server one
I unfortunately have not much time I can allocate to this
Still it would be vert cool and valuable to merge this PR
do you feel trying yourself ?
just look at the drone "Documentation" check

[camilasan]

this is a great update @joshtrichards, thanks!
I will try to make time to look into Matthieu's comments.