Updated content to conform to README best practices template

Before alterations, this content was the module author-determined
description of and instructions for use of the module.

As part of a joint Forge/Docs team effort to standardize formatting and
encourage quality module documentation, a best practices README
template was created via internal and external user testing. That
template was then applied to this module.

I pulled in content from the original README on GitHub. Standard
headings were added (Overview, Module Description, Setup, Usage,
Implementation, etc.) to organize content, existent and pulled-in
content was moved under its appropriate heading and edited for
tone/flow/clarity, and basic formatting was done to adhere to template
standards.
Updated README content to accurately reflect reworked module

Pre-changes, this README reflected the current apache module. However,
the apache module is being reworked.

This is the updated README to reflect the workings of the new module.
It is only a first draft and will have more updates as questions are
answered and development continues. (This version still contains
self-notes. )
Updated content to conform to README best practices template

This README has gone through several drafts. Initially, it was the
module author-determined description of and instructions for use of the
module. I rewrote that README to reflect the Apache module as it was on
GitHub. After a discussion with @hunner , I reworked the README to
reflect his rewriting of the module.

This reworking added sections to reflect the new structuring of the
module, fleshed out content to explain the new features, added a ton of
parameters and examples to guide usage.

This commit is still a draft and is up for review.
Fixed the TOC

I originally just completely wonked out the TOC. Fixed it.
Updated formatting and content per feedback

Updated header formatting per feedback.

Made various changes  to content per feedback

Removed a2mod since it is no longer being used.
Update class/define section headers
Change vhost to apache::vhost
Update apache::mod description
Update apache::listen description
Merge pull request #1 from hunner/update_lr_readme

Update lr readme
Renamed apache::mod::default to apache::default_mods in README.md
Updated format and layout

Rearranged subsection headers under Usage and Implementation to more
accurately, clearly and consistently reflect module flow. This
includes: replacing subsection Class: apache, Additional Classes and
Defined Types, and Defines Type: apache::vhost, with  Classes and
Defined Types; updating H-levels of subsections so classes and dts are
h4 and parameters are h5; and moving classes and dts that are not meant
to be used directly by the user to Implementation.

Updated TOC to reflect structure changes.
Added config purge notes

Added a line about the config files/directories being purged in "What
Apache affects" and added more in depth explanation under Classes and
Defined Types.
Cleaned up sections discussing vhost defaults.

Originally, discussion of vhost defaults assumed that: 1.) default
params for a vhost were set through the base apache class, and 2.) that
you could either have a default vhost through the base class OR none.
Neither of these things are true, so updated documentation to reflect
this.
Updated apache::mod::* with information about mods with params. Denoted which mods have params. Explained mods with params below list.
Fixed typos and added new vhost params

Fixed some extraneous formatting, an erroneous link and an omitted
default value.

Added `setenv` and `setenvif` to vhost param documentation.
Updated links in TOC and body to work

Links were broken
Testing to see if link works with new formatting
Updated links to new (functional) format and added 3 sections for top classes/dts

Finally figured out the correct formatting for pointing at
class/defined types listings. Updated links with those.

Added three lines to TOC to point at top 2 classes/defined types in
Usage.
Still broken. Fix attempt