[docs] update package, features and config mode docs

[maurerle]

This is a series of docs improvements created from #3131

I tested the docs using make -C docs html && python -m http.server --directory docs/_build/html.

I am open for additions and suggestions.
If I should split the commits into separate PRs I can sure do that too, though I did not want to create 5 PRs about docs.

I think that especially some visualizations to refer to are quiet important for users too, so I added two pictures.

[maurerle]

@skorpy2009 would you like to have a look at this, please?

[blocktrron]

Pleases prefix and separate the commits by topic.

[herbetom]

not a complete review, just what i noticed at a glance

[maurerle]

I would like to get this merged :)
Is there anything else I should do or improve?

[maurerle]

I update the PR to reflect to latest changes of the web-cellular package in #3307
See: https://github.com/freifunk-gluon/gluon/compare/68e431003d183ed474d9b552482f6aeeb0cb9a3c..ecc12e46d4e950c3f30e2dc8b7f9c97000aaa050

The build of the resulting docs can be found here:
https://gluon--3188.org.readthedocs.build/en/3188/ ("Details" link from the CI runs)

[rotanid]

i'd merge it, if you revert your wlan/wifi change.
wifi is a trademark and we want to use wlan.
if there is something like "wifi", please change that one in turn to "wlan", but not the other way around.

[maurerle]

Thanks @rotanid - I removed this commit through a rebase.
Lets merge?