Add overview of custom NGF policies

[kate-osborn]

Proposed changes

Add an overview of NGF policies.

Problem: There's no documentation that describes how NGF policies work.

Solution: Add a document that gives an overview of NGF custom policies. This overview includes a summary of the policies, how they are applied, how they are merged, and the format of their statuses.

Closes #1995

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[kate-osborn]

@ADubhlaoich can you help me with the table of contents for this doc? It's only showing the Overview header:

[ADubhlaoich]

My apologies for the requested changes, but the image alt text must be addressed.

https://www.w3.org/WAI/tutorials/images/decision-tree/
https://www.w3.org/WAI/tutorials/images/complex/

Beyond that (And some minor suggestions), LGTM.

[ADubhlaoich]

@ADubhlaoich can you help me with the table of contents for this doc? It's only showing the Overview header:

I think this might possibly be caused by the broken table, so we'll see how it stands after the fix.

[ADubhlaoich]

LGTM!