mkdocs uses a markdown renderer that is hardcoded to 4 spaces per tab
for detecting indentation levels, including ordered- and
unordered-lists. Since we cannot easily change the renderer, begin using
a markdown linter in CI that will fail if official docs do not adhere to
the spacing rules.

As a starting point, the markdownlint config does not begin with the
default set of checks, which might overwhelm attempts to fix them.
Instead, focus on list-tab-spacing rules and a few other highly useful
checks.

markdownlint also has some gaps in its abilities that allow common Rook
doc issues to pass acceptance. However, it allows creating custom
linting plugins. Create 2 such linting plugins to check 2 things:

- all doc lines (except code blocks) must be aligned to a 4-space
  boundary, without exception. This ensures that markdown will render
  correctly with mkdocs. This unfortunately makes it possible to create
  lists that are internally aligned strangely.
- admonitions must all follow the same format of
  ```
  !!! header
      body
  ```

For the strange lists, this is allowed and renders correctly, but it
looks strange:

```md
- first bullet
- second bullet
    still second bullet
- third bullet

    has a paragraph
    of text inside

- last bullet

Signed-off-by: Blaine Gardner <blaine.gardner@ibm.com>