[ONPREM-2555] Add docs on setting data retention policies on server

[christian-stephen]

Description

What did you change?

Added server operator docs on setting data retention policies on server. These docs are essentially a copy-paste of docs from Field Engineering: https://fieldguide.circleci-labs.com/server/data_retention/

Reasons

Why did you make these changes?

Jira: https://circleci.atlassian.net/browse/ONPREM-2555

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs):

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Include relevant backlinks to other CircleCI docs/pages.

[yaningo]

I think it's missing the associated navigation items in the respective modules/ROOT/nav.adoc.

[christian-stephen]

I think it's missing the associated navigation items in the respective modules/ROOT/nav.adoc.

This feels like it should fail with a check in CI, along with autogenerating the nav.adoc.