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>