Add precommit hook for enforcing markdown format with mdformat

.pre-commit-config.yaml

@@ -67,3 +67,13 @@ repos:
         language: system
         types:
           - text
+
+      - id: mdformat
+        name: mdformat
+        entry: mdformat
+        language: system
+        args:
+          - "--number"
+          - "--wrap=90"
+        types:
+          - markdown

tox.ini

@@ -22,13 +22,15 @@ platform = linux
 ignore_errors = true
 require_locked_deps = true
 locked_deps =
-    pre-commit
-    pre-commit-hooks
     black
     blacken-docs
-    reorder-python-imports
-    pylint
+    mdformat
+    mdformat-gfm
     mypy
+    reorder-python-imports
+    pre-commit
+    pre-commit-hooks
+    pylint
 commands =
     pre-commit run --all-files
     pylint --rcfile {toxinidir}/.pylintrc {toxinidir}/tox_poetry_installer/