Document container images workflows

[poikilotherm]

What this PR does / why we need it: Describe how images are produced from Github Actions.

TODOs:

• [ ] Make the logic in main workflow easier to read when it's skipped (Merging/...)
• [ ] Add a diagram about PR preview images from forked repos via comment
• [ ] Add some notes somewhere about implicits. For example, which kind of events we expect, how the secrets thing is basically assuring we run within IQSS bounds, how the main deploy job is skipped for PRs from forks, etc)
• [ ] ...?

Which issue(s) this PR closes:

Closes #9775

Special notes for your reviewer: None

Suggestions on how to test this: Not testable

Does this PR introduce a user interface change? If mockups are available, please link/include them here: No

Is there a release notes update needed for this change?: No

Additional documentation:

https://dataverse-guide--9776.org.readthedocs.build/en/9776/container/auto-usage.html