Add yamllint linter to lefthook config

Context

Closes #363828 (closed)

What does this MR do?

• Introduce yamllint in the lefthook config.

What it looks like

yamllint is not installed locally

$ lefthook run pre-push
Lefthook v0.7.7
RUNNING HOOKS GROUP: pre-push

[...]

  EXECUTE > yamllint
 ERROR: yamllint is not installed. Please install the pre-requisites locally: https://docs.gitlab.com/ee/development/documentation/testing.html#install-linters

[...]

SUMMARY: (done in 8.44 seconds)
✔️  docs-metadata
✔️  markdownlint
✔️  vale
✔️  danger
🥊  yamllint

yamllint is installed locally

$ lefthook run pre-push
Lefthook v0.7.7
RUNNING HOOKS GROUP: pre-push

[...]

  EXECUTE > yamllint

[...]

SUMMARY: (done in 7.44 seconds)
✔️  vale
✔️  markdownlint
✔️  yamllint
✔️  docs-metadata
✔️  danger

How to set up and validate locally

Check the error message when yamllint is not installed locally

• Checkout the branch for this MR
• Uninstall yamllint
• Run bundle exec lefthook run pre-push
  • You should see a nice error message with a link to the docs
• Install yamllint
• Run bundle exec lefthook run pre-push
  • The linting should run for yamllint.

Check that the lefthook addition works as expected

• Checkout the branch for this MR
• Follow the documentation to install yamllint locally
• Change a YAML file with a valid change
• Make a commit
• Run bundle exec lefthook run pre-push
  • You should see a successful entry for yamllint
• Change a YAML file with an invalid change
• Make a commit
• Run bundle exec lefthook run pre-push
• You should see a failed entry for yamllint

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.
• Mention this change in the #development channel
• Mention this change in EWIR

Communication

Hi all,

TL;DR: brew install yamllint 😉

In !88909 (merged), we have introduced a YAML linter to lefthook. If you are using lefthook, you will have to install yamllint locally. On a mac setup, brew install yamllint should do the trick.