readthodocs: fully specify a build environment

This is now expected by rtd so I've expanded using their example as
22.04 is one of our supported platforms. I tried to work out if there
was an easy way to re-generate a requirements.txt from our
pythondeps.toml but in the end went for the easier solution.

Cc:  <qemu-stable@nongnu.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231221174200.2693694-1-alex.bennee@linaro.org>
(cherry picked from commit b16a45bc5e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

.readthedocs.yml

@@ -5,16 +5,21 @@
 # Required
 version: 2
 
+# Set the version of Python and other tools you might need
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
 
+# We recommend specifying your dependencies to enable reproducible builds:
+# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 # We want all the document formats
 formats: all
-
-# For consistency, we require that QEMU's Sphinx extensions
-# run with at least the same minimum version of Python that
-# we require for other Python in our codebase (our conf.py
-# enforces this, and some code needs it.)
-python:
-  version: 3.6

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx==5.3.0
+sphinx_rtd_theme==1.1.1