[docs] Create documentation to describe available tooling

[andrewnicols]

What is the Moodle feature that needs documenting?

This is partially already implemented in /docs/tools but it needs to move out of there.

I'd suggest it should be in /general/development/tools and have it's own navbar link perhaps.

We need to document:

• [x] MDK - #54
• [x] Our PHP code sniffs - #54
• [ ] moodle-plugin-ci
• [ ] Our official docker images
• [ ] grunt, eslint, stylelint, and friends
• [ ] CiBoT

We may also want to consider documenting the following under tools:

• [ ] PHPUnit
• [ ] Behat

Is this documentation specific to a Moodle version?

No

Are you able to provide a patch for this?

Yes - I will create one