Docs update: move purchasing shared minutes info
### Problem to solve
Move relevant information on shared runners to new [Customers](https://docs.gitlab.com/ee/subscriptions/index.html) section.
### Proposal
First section of [Shared Runners pipeline minutes quota](https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html#shared-runners-pipeline-minutes-quota-starter-only) is admin, which is in the correct section.
Second section on viewing shared quota should be in user docs somewhere?
Add a link to users docs? but move [purchasing extra minutes](https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html#extra-shared-runners-pipeline-minutes-quota-free-only) to Customers
**Add note that minutes limit only applies to private projects.**
### Who can address the issue
@eread for you to mull over?
issue