And other docs cleanup.

Deprecated naming support is removed:

* Removed special treatment for modules in `markdown.extensions`
* Removed support for `mdx_` prefixes.

Support for Entry Point names added:

Support for "short names" are now implemented with entry points. 
Therefore all the users who call extension names as `toc` will not 
get errors as the builtin extensions all have entry points defined 
which match the old "short names" for modules in 
`markdown.extensions`. The benefit is that any extension can offer
the same support without requiring the user to manually copy a file 
to that location on the file system (way to many extension authors 
have included such instructions in their installation documentation).

The one odd thing about this is that we have been issuing a 
DeprecationWarning for short names and now they are fully supported 
again. But I think it's the right thing to do.

Support for using dot notation is not removed. After all, it was never 
deprecated. And we shouldn't "force" entry points. There are plenty of 
reasons why users may not want that and not all of them can be 
resolved by using class instances instead.

All of the following ways to load an extension are valid:

    # Class instance
    from markdown.extensions.toc import TocExtension
    markdown.markdown(src, extensions=[TocExtension()]

    # Entry point name
    markdown.markdown(src, extensions=['toc'])

    # Dot notation with class
    markdown.markdown(src, extensions=['markdown.extensions.toc:TocExtension'])

    # Dot notation without class
    markdown.markdown(src, extensions=['markdown.extensions.toc'])

Fixes #601. Merged in 6f87b32 from the md3 branch and did a lot of cleanup.

Changes include:

* Removed old docs build tool, templates, etc.

* Added MkDocs config file, etc.

* filename.txt => filename.md

* pythonhost.org/Markdown => Python-Markdown.github.io

* Markdown lint and other cleanup.

* Automate pages deployment in makefile with `mkdocs gh-deploy`

    Assumes a git remote is set up named "pages". Do

        git remote add pages https://github.com/Python-Markdown/Python-Markdown.github.io.git

    ... before running `make deploy` the first time.

The documentation uses features of Python-Markdown that are not supported on
GitHub and it's better to get a source view of the docs anyway. For example,
that way comments and bug reports can reference a specific line of a file.

Of course, it makes sense for Github to render the README, so that is left
with the `.md` file extension.

This is not a full Sphinx implementation. Just a limited implementation
which converts our markdown source filed to work with the default css for
Sphinx so it looks like Pythons docs.