Pinning python versions and dependencies for building docs

It turns out that we were pinned to a really old version of sphinx_rtd_theme (version 0.4.3).
This was probably causing rendering issues in the version hosted by read-the-docs (readthedocs/sphinx_rtd_theme#979)

.readthedocs.yaml

@@ -4,6 +4,14 @@
 
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    # the python version currently just matches the version used for building
+    # the docs with CircleCi
+    python: "3.10"
+
+
 python:
    install:
    - requirements: doc/source/requirements.txt

doc/source/requirements.txt

@@ -1,3 +1,13 @@
 # This is a pip requirements.txt file used by Read the Docs to make sure all
 # sphinx extensions have been installed.
 breathe
+
+# the following version numbers are somewhat arbitrary
+# (they just happened to be the version on 
+sphinx==5.3.0
+
+# it's important that sphinx_rtd_theme >= 0.5.1, otherwise Field Lists are
+# rendered weirdly: https://github.com/readthedocs/sphinx_rtd_theme/issues/979
+sphinx_rtd_theme==1.1.1
+
+readthedocs-sphinx-search==0.1.2