Fix mathjax in RTD build (#10889)

* Update docs.txt
* Update conf.py
* Update .readthedocs.yml

.readthedocs.yml

@@ -38,9 +38,17 @@ formats:
   - htmlzip
   #- pdf
 
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+  apt_packages:
+    - texlive-latex-extra
+    - dvipng
+    - texlive-pictures
+
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
     - requirements: requirements/docs.txt
     #- requirements: requirements.txt

requirements/docs.txt

@@ -1,9 +1,9 @@
-sphinx>=4.0,<4.1
-myst-parser
+sphinx>=4.0,<5.0
+myst-parser>=0.15
 nbsphinx>=0.8.5
 ipython[notebook]
 pandoc>=1.0
-docutils>=0.16,<0.18  # Sphinx not yet compatible with docutils >= 0.18
+docutils>=0.16
 sphinxcontrib-fulltoc>=1.0
 sphinxcontrib-mockautodoc
 https://github.com/PyTorchLightning/lightning_sphinx_theme/archive/master.zip#egg=pt-lightning-sphinx-theme