FIX Resolve final callout block issues

[GuySartorelli]

This PR fixes two final callout block issues:

1. A paragraph might not be the last item in the callout block - we should remove the bottom margin from the last element regardless of what that element is.
   E.g. see the list in a callout block in https://docs.silverstripe.org/en/5/developer_guides/templates/syntax/#variables
2. Links in callout blocks weren't being parsed, and therefore weren't rendering correctly. E.g. see "Formatting, Modifying and Casting Variables" in https://docs.silverstripe.org/en/5/developer_guides/templates/syntax/

Issue

• Incorrect display of some elements on documentation pages. Broken links. #282