Closes #90

* documenting custom header IDs
* removing rule about using one H1 element (that's in style guide now)
* clarifying thad admonitions only work when enabled

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

defining "ATX style headers" vs "setext style headers" here since it has been a topic of conversation

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

Large scale clean up of markdown code (in almost all files) to conform to markdown conventions that I observed to be used consistently in the official CiviCRM User Guide. These convensions are now documented in the markdownrules.md file

Misc > Markdown: removing TOC from page content b/c it's already available in nav menu, and including it in page content does not seem consistent with any content in user guide