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/sphinx-static/theme_overrides.css

@@ -0,0 +1,161 @@
+/* -*- coding: utf-8; mode: css -*-
+ *
+ * Sphinx HTML theme customization: read the doc
+ * Based on Linux Documentation/sphinx-static/theme_overrides.css
+ */
+
+/* Improve contrast and increase size for easier reading. */
+
+body {
+    font-family: serif;
+    color: black;
+    font-size: 100%;
+}
+
+h1, h2, .rst-content .toctree-wrapper p.caption, h3, h4, h5, h6, legend {
+    font-family: sans-serif;
+}
+
+.rst-content dl:not(.docutils) dt {
+    border-top: none;
+    border-left: solid 3px #ccc;
+    background-color: #f0f0f0;
+    color: black;
+}
+
+.wy-nav-top {
+    background: #802400;
+}
+
+.wy-side-nav-search input[type="text"] {
+    border-color: #f60;
+}
+
+.wy-menu-vertical p.caption {
+    color: white;
+}
+
+.wy-menu-vertical li.current a {
+    color: #505050;
+}
+
+.wy-menu-vertical li.on a, .wy-menu-vertical li.current > a {
+    color: #303030;
+}
+
+.fa-gitlab {
+      box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2), 0 3px 10px 0 rgba(0,0,0,0.19);
+      border-radius: 5px;
+}
+
+div[class^="highlight"] pre {
+    font-family: monospace;
+    color: black;
+    font-size: 100%;
+}
+
+.wy-menu-vertical {
+    font-family: sans-serif;
+}
+
+.c {
+    font-style: normal;
+}
+
+p {
+    font-size: 100%;
+}
+
+/* Interim: Code-blocks with line nos - lines and line numbers don't line up.
+ * see: https://github.com/rtfd/sphinx_rtd_theme/issues/419
+ */
+
+div[class^="highlight"] pre {
+    line-height: normal;
+}
+.rst-content .highlight > pre {
+    line-height: normal;
+}
+
+/* Keep fields from being strangely far apart due to inheirited table CSS. */
+.rst-content table.field-list th.field-name {
+    padding-top: 1px;
+    padding-bottom: 1px;
+}
+.rst-content table.field-list td.field-body {
+    padding-top: 1px;
+    padding-bottom: 1px;
+}
+
+@media screen {
+
+    /* content column
+     *
+     * RTD theme's default is 800px as max width for the content, but we have
+     * tables with tons of columns, which need the full width of the view-port.
+     */
+
+    .wy-nav-content{max-width: none; }
+
+    /* table:
+     *
+     *   - Sequences of whitespace should collapse into a single whitespace.
+     *   - make the overflow auto (scrollbar if needed)
+     *   - align caption "left" ("center" is unsuitable on vast tables)
+     */
+
+    .wy-table-responsive table td { white-space: normal; }
+    .wy-table-responsive { overflow: auto; }
+    .rst-content table.docutils caption { text-align: left; font-size: 100%; }
+
+    /* captions:
+     *
+     *   - captions should have 100% (not 85%) font size
+     *   - hide the permalink symbol as long as link is not hovered
+     */
+
+    .toc-title {
+        font-size: 150%;
+        font-weight: bold;
+    }
+
+    caption, .wy-table caption, .rst-content table.field-list caption {
+        font-size: 100%;
+    }
+    caption a.headerlink { opacity: 0; }
+    caption a.headerlink:hover { opacity: 1; }
+
+    /* Menu selection and keystrokes */
+
+    span.menuselection {
+        color: blue;
+        font-family: "Courier New", Courier, monospace
+    }
+
+    code.kbd, code.kbd span {
+        color: white;
+        background-color: darkblue;
+        font-weight: bold;
+        font-family: "Courier New", Courier, monospace
+    }
+
+    /* fix bottom margin of lists items */
+
+    .rst-content .section ul li:last-child, .rst-content .section ul li p:last-child {
+          margin-bottom: 12px;
+    }
+
+    /* inline literal: drop the borderbox, padding and red color */
+
+    code, .rst-content tt, .rst-content code {
+        color: inherit;
+        border: none;
+        padding: unset;
+        background: inherit;
+        font-size: 85%;
+    }
+
+    .rst-content tt.literal,.rst-content tt.literal,.rst-content code.literal {
+        color: inherit;
+    }
+}