docs: Add module dependency/requirement for RtD

myst_parser is not in the default Read The Docs container by default.
Pull it in as we use it for parsing the markdown files.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

.readthedocs.yaml

@@ -0,0 +1,6 @@
+version: 2
+
+python:
+  version: "3.7"
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1 @@
+myst_parser