docs: set site_url to fix broken 404 page on RTD

[lars-reimann]

Closes #443

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	0.1s
✅ YAML	prettier	1	0	0	0.56s
✅ YAML	v8r	1		0	2.13s
✅ YAML	yamllint	1		0	0.23s

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[codecov]

Codecov Report

Merging #176 (7909751) into main (44005f4) will not change coverage.
The diff coverage is n/a.

❗ Current head 7909751 differs from pull request most recent head 1b59d5b. Consider uploading reports for the commit 1b59d5b to get more accurate results

@@            Coverage Diff            @@
##              main      #176   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           48        48           
  Lines         2514      2514           
=========================================
  Hits          2514      2514           

[lars-reimann]

🎉 This PR is included in version 0.15.0 🎉

The release is available on:

• v0.15.0
• GitHub release

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