Support for a hand-written Quick Reference

[alandefreitas]

Mr. Docs needs to support a hand-written Quick Reference.

This is a Mr. Docs issue because it is a blocker. This could be an independent page, but if the hyperlink looks like this:

[mrdocs]xref:x01234923t9xzvq1244

then we are going to have a problem.

Mr. Docs needs an example that demonstrates this, and it needs to be rendered as part of the tests and PR/master/develop doc previews so we can ensure it works.

Mr. Docs may not need any work, but MVP requires this, so it is a Mr. Docs issue. The issue may be closed if we prove it can be done currently.

If they become complicated, such as in the case of template specializations and overloads, then we might need a Mr. Docs feature for the Adoc generator to emit a separate file containing an alphabetical index of adoc links to all symbols and their fully qualified names

[vinniefalco]

The docs/tests for Mr. Docs need to have an example of this so we can preview that it is still working