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/conf.py b/docs/conf.py
index 356be99..ef77f6b 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -20,13 +20,13 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['sphinx.ext.autosectionlabel', 'sphinxcontrib.plantuml']
+extensions = ['myst_parser', 'sphinx.ext.autosectionlabel', 'sphinxcontrib.plantuml']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
 
 # The suffix(es) of source filenames.
-source_suffix = '.rst'
+source_suffix = ['.md', '.rst']
 
 # The master toctree document.
 master_doc = 'index'