Add Lefthook to the Gemfile and improve documentation

What does this MR do?

I think Lefthook has proven to work well, and I don't see a good reason not to add it to the development dependencies (similarly to how we install it in the GDK: gitlab-development-kit!1500 (diffs)).

I've also improved the documentation by describing the current configuration.

mentioned in issue gitlab-development-kit#1149 (closed)

changed milestone to %13.9

added Engineering Productivity maintenanceworkflow static code analysis labels

added backend documentation labels

added tooling (archive) label

	1 Message
	This merge request adds or changes documentation files. A review from the Technical Writing team before you merge is recommended. Reviews can happen after you merge.

Documentation review

The following files require a review from a technical writer:

• doc/development/contributing/style_guides.md
• doc/development/documentation/testing.md

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Category	Reviewer	Maintainer
backend	Małgorzata Ksionek (@mksionek) (UTC+1, same timezone as @rymai)	Heinrich Lee Yu (@engwan) (UTC+8, 7 hours ahead of @rymai)

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• cda2d0fc - Add Lefthook to the Gemfile and improve documentation

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• b54cecd4 - Add Lefthook to the Gemfile and improve documentation

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@ashmckenzie Could you please review? Thanks!

assigned to @ashmckenzie and unassigned @rymai

mentioned in merge request !52114 (merged)

assigned to @engwan and unassigned @ashmckenzie

approved this merge request

Thanks @rymai, code change looks good to me

@eread can you review this please since you reviewed the initial version of this?