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?

Update markdownrules.md

API: Fix more non-standard examples

Importing "Book Style Guide" wiki page from https://wiki.civicrm.org/confluence/display/CRM/Book+style+guide

indent comments in yml

update from civi

The most canonical form is to present entities in camel-case and and to
present actions in lower-case.  This matches the API explorer.

API: code examples broken or with non-standard style

small edits to Erich's recent PR

CRMDOC/Create+an+Extension

This completes transfer of

https://wiki.civicrm.org/confluence/display/CRMDOC/Create+an+Extension

This page is yet to do:

https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Module+Extension

update from core

use hash syntax for headings, migrate "developer community" wiki page

https://wiki.civicrm.org/confluence/display/CRMDOC/The+developer+community

perl -0777 -i -pe 's|^(.*)\n-{3,}$|## $1|gm' `find . -name '*.md'`
perl -0777 -i -pe 's|^(.*)\n={3,}$|# $1|gm' `find . -name '*.md'`

New content in markdown.md page and lots of cleaning-up of other content

INFRA-230 - Add rewrite map to facilitate wiki migration