Updates to Contributor's Guide

Updates to documentation Contributor's Guide following documentation style and tone discussions.

Don’t use link: at the start of link macros if the URL makes it clear
If the URL is repeated on the page, or is extraordinarily long, use a doc attribute for the URL
External links should all have ^
when using callouts, instead of using the <1>, <2> notation, we can instead use <.>. This method auto numbers the callouts
The callout example in the contributor's guide should be marked AsciiDoc