markdownlint: fixes

The protocol spec has some custom HTML to create tables-in-tables,
which do not have an equivalent markdown syntax.  Copy the reference
markdownlint config from openbmc-build-scripts and tweak to allow those
additional references.

Do minor changes elsewhere to fix any outstanding markdownlint warnings.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If8811c711dbb3fb05b4d24b1fb2b705c4cd36826
diff --git a/.markdownlint.yaml b/.markdownlint.yaml
new file mode 100644
index 0000000..b3d9dd0
--- /dev/null
+++ b/.markdownlint.yaml
@@ -0,0 +1,6 @@
+default: true
+MD013: false
+MD024:
+    siblings_only: true
+MD033:
+    allowed_elements: ["br", "sub", "sup", "table", "tr", "th", "td"]