Document Shared Runners minutes calc

What does this MR do?

Describes how Shared Runners' minutes quota is applied to gitlab.com and on-premise installs.
Mentions upcoming public project minutes tracking.
For now, 13.0 mentioned as a target milestone when the change will be active.
It could be changed according to our release process.

Related to this, we need also to change https://about.gitlab.com/pricing/, Do limits apply to public and private projects? in QA section. Will be in a separate MR.

Related to #210570 (closed)

Related issues

Related to #213096 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

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3. Maintainer

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