Update install docs.

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.