Draft: rename markdownlint configuration file

What does this MR do?

This changes the name of the markdownlint-cli config file, and updates this change throughout the project.

According to the markdownlint-cli docs, the program will look for certain configuration files in the current folder (.markdownlint.json/.markdownlint.yaml/.markdownlint.yml), but will look for a .markdownlintrc in any parent folder.

In general, making this fix should reduce some minimal friction in setting up a workflow for people working on GitLab documentation. For example, if you use Flycheck, this fix allows it to work out of the box in emacs.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.