Add Image Pull Policy Docs

This is an initial skeleton of the docs, a lot of TBDs that will be resolved by other issues in the epic.

Preview: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/micah.nagel-master-patch-60517/docs/guides/using_bigbang/image_pull_policy.md

Closes https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/915