doc: Pin dependencies

Something changed recently with dependency resolution on ReadTheDocs.
For some reason it's now pulling sphinx-rtd-theme 0.5.1, whereas before
it was pulling 2.0.0.  Not sure what's going on, but hopefully pinning
the versions of sphinx and sphinx-rtd-theme does the trick.

doc/requirements.txt

@@ -1,8 +1,8 @@
 # Requirements for building the `shell-logger` documentation.
 
-sphinx
+sphinx>=8.0.0
 sphinx-argparse
 sphinx-autodoc-typehints
 sphinx-copybutton
-sphinx-rtd-theme
+sphinx-rtd-theme>=2.0.0
 sphinxcontrib-programoutput