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

INFRA-230 - Add rewrite map to facilitate wiki migration

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

Documentation about the documentation 

tweaks

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

gender-inclusive pronoun for extension devs

During the meeting of the Extension Working group on 27-Oct-16, we discussed
trade-offs in the names.  (Ex: Now that "Stewardship" has an option for
"Seeking Maintainer", it seems a bit confusing to have "Maturity" as
"Maintained/Unmaintained".) There was support changing these two names and
no opposition.

The goal is to publicize this soon in a public blog.  Hopefully, this is the
last round, but if there's strong feedback we could revise again.

extend-stages.md - Fix table layout in Github

The table layout looks right with `mkdocs` but not with Github. This works in both.

Add "Extension Lifecycle" chapter

The `first_name`/`organiztion_name` is kind of an ugly bit and tangential to
the main point here (entities vs actions vs parameters).  Choose fields that
more clearly apply to all contacts.

Assorted documentation improvements.