Create rtd config file, split requirements, and pin versions

This does exactly what we did to fix the rtd builds over in [1].

[1] - simonsobs/socs#209

.readthedocs.yaml

@@ -0,0 +1,10 @@
+version: 2
+
+sphinx:
+    configuration: docs/conf.py
+
+python:
+    version: 3.7
+    install:
+    - requirements: docs/requirements.txt
+    - requirements: requirements.txt

README.rst

@@ -88,10 +88,12 @@ be considered unstable.
 
 Documentation
 -------------
-The OCS documentation can be built using sphinx once you have performed the
-installation::
+The OCS documentation can be built using Sphinx. There is a separate
+``requirements.txt`` file in the ``docs/`` directory to install Sphinx and any
+additional documentation dependencies::
 
   $ cd docs/
+  $ pip3 install -r requirements.txt
   $ make html
 
 You can then open ``docs/_build/html/index.html`` in your preferred web

docs/requirements.txt

@@ -0,0 +1,3 @@
+sphinx==4.2.0
+sphinx_rtd_theme==1.0.0
+sphinx-argparse==0.3.1

requirements.txt

@@ -12,9 +12,7 @@ numpy
 pyserial
 
 # Documentation generation
-sphinx
-sphinx-argparse
-sphinx_rtd_theme
+# see docs/requirements.txt
 
 # checkdata
 progress