Switch to requirements.txt for readthedocs.

Bulleted lists were not rendering properly. Apparently, this is
an old bug (readthedocs/sphinx_rtd_theme#1115)
that has recently resurfaced. The solution is to pin newer versions
of sphinx and sphinx_rtd_theme (https://stackoverflow.com/questions/67542699/readthedocs-sphinx-not-rendering-bullet-list-from-rst-file/71069918#71069918).
However, readthedocs conda build environment does not allow pinning:
readthedocs/readthedocs.org#3829

Thus, the only solution is to convert to a requirements.txt build
on readthedocs. While configuring this build, I also updated to the
latest system image, python version, and to a non-deprecated
name for the readthedocs configuration file
(https://docs.readthedocs.io/en/stable/config-file/v2.html).

Author: joaander

readthedocs.yml renamed to .readthedocs.yaml

@@ -1,7 +1,8 @@
 version: 2
 
-conda:
-    environment: sphinx-doc/environment.yml
+python:
+   install:
+   - requirements: sphinx-doc/requirements.txt
 
 sphinx:
   configuration: sphinx-doc/conf.py
@@ -11,9 +12,9 @@ formats:
   - htmlzip
 
 build:
-  os: ubuntu-20.04
+  os: ubuntu-22.04
   tools:
-    python: mambaforge-4.10
+    python: "3.11"
 
 submodules:
   include: all

sphinx-doc/environment.yml

@@ -0,0 +1,10 @@
+h5py
+ipython
+ipython_genutils
+nbconvert
+nbsphinx
+numpy
+pandoc
+sphinx>=4.3.0
+sphinx-rtd-theme>=0.5.1
+tornado