sphinx-static/theme_overrides.css: switch to a fixed width documentation

Change the css rules of our theme to have a fixed-width documentation
instead of it taking the full width of the page. I believe this makes it
much more readable compared to having long lines with few line returns,
especially on high-resolution displays. Set the width to 1000px instead
of the previous 800px, which felt slightly too thin.

I think the removed comment here does not make that much sense for us.
It was added by commit 0c1e108bc6c4 ("sphinx: add CSS theme override"),
and I believe is a simply copy and paste of what was is set in the Linux
kernel, added by commit 9abaf979abb2 ("doc-rst: customize RTD theme,
table & full width") [1].

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9abaf979abb2

(From yocto-docs rev: 680edf7ffdf2286c64c32de74be5b6353294122f)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 98234c9d3a0846d719630914bea8599da9f51374)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Paul Barker <paul@pbarker.dev>

documentation/sphinx-static/theme_overrides.css

@@ -99,14 +99,19 @@ em {
 [alt='Permalink'] { color: #eee; }
 [alt='Permalink']:hover { color: black; }
 
-@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.
-     */
+.literal-block {
+    background: #f8f8f8;
+}
 
-    .wy-nav-content{max-width: none; }
+@media screen {
+    .wy-nav-content {
+       max-width: 1000px;
+       background: #fcfcfc;
+    }
+
+    .wy-nav-content-wrap {
+       background: #efefef;
+    }
 
     /* inline literal: drop the borderbox, padding and red color */
     code, .rst-content tt, .rst-content code {