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)

.markdownlint.yaml

----
-MD013: false
-MD029: false

.md_style.rb

+all
+exclude_rule 'MD013'
+exclude_rule 'MD029'
+rule 'MD007', :indent => 2

.mdlrc

+style "#{File.dirname(__FILE__)}/.md_style.rb"

.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

docs/cloud_providers/openstack.md

-
 # OpenStack
 
 ## Known compatible public clouds

docs/operations/recover-control-plane.md

-
 # Recovering the control plane
 
 To recover from broken nodes in the control plane use the "recover\-control\-plane.yml" playbook.