[requirements-docs.txt]: Update sphinx-rtd-theme >=3.0, <4.0

Update sphinx-rtd-theme from `>=1.0, <4.0` to `>=3.0, <4.0`.

In version 3.0.0, the configuration option
[`display_version`](https://sphinx-rtd-theme.readthedocs.io/en/3.0.0/configuring.html#confval-display_version)
was replaced by
[`version_selector`](https://sphinx-rtd-theme.readthedocs.io/en/3.0.0/configuring.html#confval-version_selector).
Note that
[`version_selector`](https://sphinx-rtd-theme.readthedocs.io/en/3.0.0/configuring.html#confval-version_selector)
requires the documentation to be hosted on
[Read the Docs](https://readsthedocs.com/),
because it makes use of
[Read the Docs Addons](https://docs.readthedocs.io/page/addons.html).

Author: andthum

docs/requirements-docs.txt

@@ -1,5 +1,5 @@
 # Requirements to build hpcss' documentation.
 
 sphinx >=5.0, <9.0
-sphinx-rtd-theme >=1.0, <4.0
+sphinx-rtd-theme >=3.0, <4.0
 tomlkit >=0.1, < 1.0