markdown rules - specifying that internal hyperlinks should use absolute paths with preceding slashes

1) This seems like a site admin task, not a developer task. If you're reading this page, I assume you
   already know how to do this. I'd argue this belongs in the user guide (if it's not already there).
2) It doesn't seem right to me that one should configure their extensions directory before
   deciding whether or not to write an extension. First check the extensions directory to
   see if someone has already built the thing. If not, only then does make sense to start
   playing with your environment config.
3) If we keep it in the developer guide, I'd put it at the beginning of the instructions
   for creating a module extension (https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Module+Extension).
   These are currently undergoing migration from the wiki to mkdocs, so I'm blocked from making such
   an update at this time.

 * Cleanup syntax of `define()`s. Add another.
 * Add `angularDebug=1` / `crm-ui-debug`.
 * Add more ways to clear the cache.
 * Consolidate discussions of xdebug (for all other/undocumented/external)

https://wiki.civicrm.org/confluence/display/CRMDOC/Debugging+for+developers

https://wiki.civicrm.org/confluence/display/CRMDOC/Recommendations
(and wiki redirect to go with it)

https://wiki.civicrm.org/confluence/display/CRMDOC/Before+you+start

and more tidying up and grammar etc

Style guide: removing bit about all caps -- couldn't find an example of that in practice in the User Guide

Honestly I don't understand what this section is asking documentation
editors to do. If want this section back in, perhaps you could add some
clarification?