pre-commit: adjust mardownlint default, md fixes

Use a style file as recommended by upstream. This makes for only one
source of truth.
Conserve previous upstream default for MD007 (upstream default changed
here https://github.com/markdownlint/markdownlint/pull/373)

.pre-commit-config.yaml

@@ -24,8 +24,7 @@ repos:
     rev: v0.11.0
     hooks:
       - id: markdownlint
-        args: [-r, "~MD013,~MD029"]
-        exclude: "^.git"
+        exclude: "^.github|(^docs/_sidebar\\.md$)"
 
   - repo: https://github.com/shellcheck-py/shellcheck-py
     rev: v0.10.0.1