Add project specific maintainership

Why is this change being made?

There are engineers which focus on specific projects within GitLab, and while they may have expert level knowledge of those specific projects, there isn't clear guidance on becoming a maintainer for specific project. Also as we continue to add engineers the need for more maintainers is increasing and should promote ways for engineers to become maintainers.

Existing wording in this section hints at project specific maintainership -- specifically, "Every project has at least one maintainer, but most have multiple." But there is not clear route for becoming a maintainer of a specific application.

I also think we should consider project specific maintainership as the route for becoming a maintainer in general. Out of demand, all Frontend GitLab project maintainers have become maintainers of the Customers Portal, but going forward we should consider add maintainers on project per project basis. This would require significant re-writing of this section.

Also to address the concern of projects deviating from corporate standards I recommend having project specific maintainer reviews reviewed by outside maintainers 5% of the time (although 5% is just an example and that bar could set arbitrarily higher or lower)

Relates to: !43046 (merged)

How will this merge request impact page speed?

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what