BUG: Specify the language in Sphinx documentation config file

Specify the language in Sphinx documentation config file.

Fixes:
```
Running Sphinx v7.2.6

Warning, treated as error:
Invalid configuration value found: 'language = None'.
Update your configuration to a valid language code.
Falling back to 'en' (English).
make: *** [Makefile:65: html] Error 2
```

raised when running:
```
make -C docs/ SPHINXOPTS="-W" BUILDDIR="$HOME/docs" OUTDIR="${CURBRANCH:-html}" html
```

docs/conf.py

@@ -91,7 +91,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = "en"
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.