Docs update: move purchasing shared minutes info

Problem to solve

Move relevant information on shared runners to new Customers section.

Proposal

First section of Shared Runners pipeline minutes quota is admin, which is in the correct section.

Section section on viewing shared quota should be in user docs somewhere?

Add a link to users docs? but move purchasing extra minutes to Customers

Add note that minutes limit only applies to private projects.

Who can address the issue

@eread for you to mull over?