docs: Switch the default HTML theme to alabaster

The read-the-docs theme is not entirely attractive and doesn't give us
control over the left column.  "Alabaster" is deemed the default Sphinx
theme, it is currently maintained and shipped bundled with Sphinx itself,
so there is no need to install it separately.  Switch over to this theme as
the default for building kernel documentation; the DOCS_THEME environment
variable can still be used to select a different theme.
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -194,6 +194,24 @@ finally:
     else:
         version = release = "unknown version"
 
+#
+# HACK: there seems to be no easy way for us to get at the version and
+# release information passed in from the makefile...so go pawing through the
+# command-line options and find it for ourselves.
+#
+def get_cline_version():
+    c_version = c_release = ''
+    for arg in sys.argv:
+        if arg.startswith('version='):
+            c_version = arg[8:]
+        elif arg.startswith('release='):
+            c_release = arg[8:]
+    if c_version:
+        if c_release:
+            return c_version + '-' + c_release
+        return c_version
+    return version # Whatever we came up with before
+
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
 #
@@ -247,7 +265,7 @@ highlight_language = 'none'
 # a list of builtin themes.
 
 # Default theme
-html_theme = 'sphinx_rtd_theme'
+html_theme = 'alabaster'
 html_css_files = []
 
 if "DOCS_THEME" in os.environ:
@@ -324,6 +342,10 @@ if  html_theme == 'classic':
         'bodyfont':            "serif",
         'headfont':            "sans-serif",
     }
+else:
+    html_theme_options = {
+        'description': get_cline_version(),
+    }
 
 sys.stderr.write("Using %s theme\n" % html_theme)
 
@@ -370,8 +392,8 @@ html_static_path = ['sphinx-static']
 html_use_smartypants = False
 
 # Custom sidebar templates, maps document names to template names.
-# Note that the RTD theme ignores this.
-html_sidebars = { '**': ['searchbox.html', 'localtoc.html', 'sourcelink.html']}
+# Note that the RTD theme ignores this
+html_sidebars = { '**': ["about.html", 'searchbox.html', 'localtoc.html', 'sourcelink.html']}
 
 # Additional templates that should be rendered to pages, maps page names to
 # template names.