Switch docs to MKDocs (#602)

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.

1 files changed, 45 insertions, 0 deletions

diff --git a/docs/extensions/abbreviations.md b/docs/extensions/abbreviations.md
new file mode 100644
index 0000000..d580d56
--- /dev/null
+++ b/docs/extensions/abbreviations.md
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+title: Abbreviations Extension
+
+Abbreviations
+=============
+
+Summary
+-------
+
+The Abbreviations extension adds the ability to define abbreviations.
+Specifically, any defined abbreviation is wrapped in  an `<abbr>` tag.
+
+The Abbreviations extension is included in the standard Markdown library.
+
+Syntax
+------
+
+Abbreviations are defined using the syntax established in
+[PHP Markdown Extra][php].
+
+[php]: http://www.michelf.com/projects/php-markdown/extra/#abbr
+
+Thus, the following text (taken from the above referenced PHP documentation):
+
+```md
+The HTML specification
+is maintained by the W3C.
+
+*[HTML]: Hyper Text Markup Language
+*[W3C]:  World Wide Web Consortium
+```
+
+will be rendered as:
+
+```html
+<p>The <abbr title="Hyper Text Markup Language">HTML</abbr> specification
+is maintained by the <abbr title="World Wide Web Consortium">W3C</abbr>.</p>
+```
+
+Usage
+-----
+
+See [Extensions](index.md) for general extension usage, specify `markdown.extensions.abbr`
+as the name of the extension.
+
+This extension does not accept any special configuration options.