Prepare template documents for furo style docs

[fwitte]

Resolve #1074

This PR prepares the change of the docs layout to the furo template. The following things have to considered before merging:

• Check for compatibility of the old docs pages to the new template
• Create darkmode replacements for every figure and logo
• Insert darkmode/lightmode responsive figure inclusion code
• Check if the docs actually build on readthedocs

On top of that, I updated the .readthedocs.yml configuration file to use the [dev] dependencies and a more up to date python version and delete the extra readthedocs specific requirements.txt file. It also introduces an announcement banner at the top of every page, where the content can be dynamically changed from the announcement.html file in the root of the announcements branch.

The plan is to crowdsource the necessary updates at the upcoming user-meeting.

[pep8speaks]

Hello @fwitte! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file docs/conf.py:

Line 100:1: E265 block comment should start with '# '
Line 129:80: E501 line too long (122 > 79 characters)

[fwitte]

I added instructions to contribute to this change for the in person meeting in the respective issue #1074