docs: set site_url to fix broken 404 page on RTD

[lars-reimann]

Summary of Changes

The 404 page was previously not properly rendered on Read the Docs (RTD). This PR overrides the build commands of RTD. While this makes the 404 page work properly, it also removes the RTD flyout to select a version. For now, however, we deem the 404 page as more essential.

Once readthedocs/readthedocs.org#8529 is done, we can use the default build again to make everything work as expected.

[lars-reimann]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ REPOSITORY	git_diff	yes		no	0.62s
✅ YAML	prettier	1	0	0	1.13s
✅ YAML	v8r	1		0	2.72s
✅ YAML	yamllint	1		0	0.27s

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[codecov]

Codecov Report

Merging #1325 (474ac40) into main (e39d83d) will increase coverage by 10.73%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main    #1325       +/-   ##
===========================================
+ Coverage   57.01%   67.75%   +10.73%     
===========================================
  Files          86       40       -46     
  Lines        7091     1907     -5184     
  Branches      668      371      -297     
===========================================
- Hits         4043     1292     -2751     
+ Misses       2693      551     -2142     
+ Partials      355       64      -291     

Components	Coverage Δ
backend	81.66% <ø> (ø)
desktop	2.67% <ø> (ø)
gui	∅ <ø> (∅)

see 46 files with indirect coverage changes

[lars-reimann]

🎉 This PR is included in version 1.93.0 🎉

The release is available on GitHub release

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