Enable $$ blocks inline via myst parser dmath_double_inline (#658)

Refs:
- Myst docs: https://myst-parser.readthedocs.io/en/latest/configuration.html#id1:~:text=default%3A%20True)-,dmath_double_inline,-bool
- Changelog: https://myst-parser.readthedocs.io/en/latest/develop/_changelog.html#id19
- PR adding this option to myst: executablebooks/MyST-Parser#369
- Relevant issue from related project: jupyter-book/jupyter-book#1181

Fixes #657

Author: isms

examples/conf.py

@@ -135,6 +135,7 @@ def setup(app: Sphinx):
 
 # MyST config
 myst_enable_extensions = ["colon_fence", "deflist", "dollarmath", "amsmath", "substitution"]
+myst_dmath_double_inline = True
 citation_code = f"""
 ```bibtex
 @incollection{{citekey,