sphinx: adopt kernel readthedoc theme

The default "alabaster" sphinx theme has a couple shortcomings:
- the navbar moves along the page
- the search bar is not always at the same place
- it lacks some contrast and colours

The "rtd" theme from readthedocs.org is a popular third party theme used
notably by the kernel, with a custom style sheet. I like it better,
perhaps others do too. It also simplifies the "Edit on Gitlab" links.

Tweak a bit the custom theme to match qemu.org style, use the
QEMU logo, and favicon etc.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20210323115328.4146052-1-marcandre.lureau@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

docs/conf.py

@@ -29,6 +29,7 @@
 import os
 import sys
 import sphinx
+from distutils.version import LooseVersion
 from sphinx.errors import ConfigError
 
 # Make Sphinx fail cleanly if using an old Python, rather than obscurely
@@ -150,38 +151,47 @@ with open(os.path.join(qemu_docdir, 'defs.rst.inc')) as f:
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
-html_theme = 'alabaster'
+try:
+    import sphinx_rtd_theme
+except ImportError:
+    raise ConfigError(
+        'The Sphinx \'sphinx_rtd_theme\' HTML theme was not found.\n'
+    )
+
+html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.
-# We initialize this to empty here, so the per-manual conf.py can just
-# add individual key/value entries.
-html_theme_options = {
-}
+if LooseVersion(sphinx_rtd_theme.__version__) >= LooseVersion("0.4.3"):
+    html_theme_options = {
+        "style_nav_header_background": "#802400",
+    }
+
+html_logo = os.path.join(qemu_docdir, "../ui/icons/qemu_128x128.png")
+
+html_favicon = os.path.join(qemu_docdir, "../ui/icons/qemu_32x32.png")
 
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-# QEMU doesn't yet have any static files, so comment this out so we don't
-# get a warning about a missing directory.
-# If we do ever add this then it would probably be better to call the
-# subdirectory sphinx_static, as the Linux kernel does.
-# html_static_path = ['_static']
+html_static_path = [os.path.join(qemu_docdir, "sphinx-static")]
+
+html_css_files = [
+    'theme_overrides.css',
+]
+
+html_context = {
+    "display_gitlab": True,
+    "gitlab_user": "qemu-project",
+    "gitlab_repo": "qemu",
+    "gitlab_version": "master",
+    "conf_py_path": "/docs/", # Path in the checkout to the docs root
+}
 
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.
-#
-# This is required for the alabaster theme
-# refs: http://alabaster.readthedocs.io/en/latest/installation.html#sidebars
-html_sidebars = {
-    '**': [
-        'about.html',
-        'editpage.html',
-        'navigation.html',
-        'searchbox.html',
-    ]
-}
+#html_sidebars = {}
 
 # Don't copy the rST source files to the HTML output directory,
 # and don't put links to the sources into the output HTML.