build(docs): add support for Markdown documentation

This changes adds support for building Markdown documentation into
Sphinx with [MyST]. We'll make use of this in a later patch, where we
introduce automatically-generated Markdown documentation that needs to
be compiled as part of the Sphinx documentation.

[MyST]: https://myst-parser.readthedocs.io/en/latest

Change-Id: I2a241a588c579fac1a81e1853479908928be1fc8
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/requirements.in b/docs/requirements.in
index 192310d..5d771e5 100644
--- a/docs/requirements.in
+++ b/docs/requirements.in
@@ -1,3 +1,4 @@
+myst-parser==0.15.2
 pip-tools==6.4.0
 sphinx==4.2.0
 sphinx-rtd-theme==1.0.0