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>

tests/docker/dockerfiles/alpine.docker

@@ -39,6 +39,7 @@ ENV PACKAGES \
 	pulseaudio-dev \
 	python3 \
 	py3-sphinx \
+	py3-sphinx_rtd_theme \
 	shadow \
 	snappy-dev \
 	spice-dev \

tests/docker/dockerfiles/debian10.docker

@@ -32,6 +32,7 @@ RUN apt update && \
         psmisc \
         python3 \
         python3-sphinx \
+        python3-sphinx-rtd-theme \
         $(apt-get -s build-dep --arch-only qemu | egrep ^Inst | fgrep '[all]' | cut -d\  -f2)
 
 ENV FEATURES docs

tests/docker/dockerfiles/fedora.docker

@@ -92,6 +92,7 @@ ENV PACKAGES \
     python3-pillow \
     python3-pip \
     python3-sphinx \
+    python3-sphinx_rtd_theme \
     python3-virtualenv \
     rdma-core-devel \
     SDL2-devel \

tests/docker/dockerfiles/ubuntu.docker

@@ -63,6 +63,7 @@ ENV PACKAGES \
     ninja-build \
     python3-yaml \
     python3-sphinx \
+    python3-sphinx-rtd-theme \
     sparse \
     xfslibs-dev
 RUN apt-get update && \

tests/docker/dockerfiles/ubuntu1804.docker

@@ -48,6 +48,7 @@ ENV PACKAGES \
     make \
     python3-yaml \
     python3-sphinx \
+    python3-sphinx-rtd-theme \
     ninja-build \
     sparse \
     xfslibs-dev

tests/docker/dockerfiles/ubuntu2004.docker

@@ -58,6 +58,7 @@ ENV PACKAGES flex bison \
     python3-pil \
     python3-pip \
     python3-sphinx \
+    python3-sphinx-rtd-theme \
     python3-venv \
     python3-yaml \
     rpm2cpio \