Create macro that can be called from within content to pull in child pages

[ryangyoung]

Create a macro that enables us to pull in child pages (child page title link and blurb) that can be called from within a content block.

This will enable us to provide the autogenerated child page content - within a content block that provides a bit of content that includes markdown.

For an example of a page that could benefit from this macros, see https://developers.ripple.com/ledger-object-types.html.

Original jira ticket: https://ripplelabs.atlassian.net/browse/DOC-1564