Documentation: Fix conf.py for newer Sphinx versions

Newer versions of Sphinx complain about the language being set to None,
so explicitly set it to 'en'. The syntax that is currently used to
enable custom CSS styling for tables [1] also no longer works, resulting
in the docs rendering without CSS. Fix this using the html_css_files
option instead.

TEST: The documentation builds and renders correctly with both Sphinx
1.8.3 in from the doc.coreboot.org Docker container and Sphinx 7.2.2
from distro packages.

[1] Commit a78e66e5f4: Documentation: Add static CSS file to fix tables

Change-Id: I036b1cad3cfa533c0c3a037bac649caa2d968d4b
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75466
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

Documentation/conf.py

@@ -55,7 +55,7 @@ else:
 #
 # 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.
@@ -87,11 +87,9 @@ html_theme = 'sphinx_rtd_theme'
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
-html_context = {
-      'css_files': [
-        '_static/theme_overrides.css',  # override wide tables in RTD theme
-      ],
-}
+html_css_files = [
+    'theme_overrides.css',  # override wide tables in RTD theme
+]
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'corebootdoc'