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	0.03s
✅ YAML	prettier	1	0	0	0.43s
✅ YAML	v8r	1		0	1.66s
✅ YAML	yamllint	1		0	0.19s

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[codecov]

Codecov Report

Merging #89 (ce3f703) into main (29abdb7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #89   +/-   ##
=======================================
  Coverage   91.33%   91.33%           
=======================================
  Files          87       87           
  Lines        4524     4524           
=======================================
  Hits         4132     4132           
  Misses        392      392