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, 84 insertions, 0 deletions

diff --git a/docs/extensions/admonition.md b/docs/extensions/admonition.md
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+++ b/docs/extensions/admonition.md
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+title: Admonition Extension
+
+Admonition
+==========
+
+Summary
+-------
+
+The Admonition extension adds [rST-style][rST] admonitions to Markdown documents.
+
+This extension is included in the standard Markdown library.
+
+[rST]: http://docutils.sourceforge.net/docs/ref/rst/directives.html#specific-admonitions
+
+Syntax
+------
+
+Admonitions are created using the following syntax:
+
+```md
+!!! type "optional explicit title within double quotes"
+    Any number of other indented markdown elements.
+
+    This is the second paragraph.
+```
+
+`type` will be used as the CSS class name and as default title. It must be a
+single word. So, for instance:
+
+```md
+!!! note
+    You should note that the title will be automatically capitalized.
+```
+
+will render:
+
+```html
+<div class="admonition note">
+<p class="admonition-title">Note</p>
+<p>You should note that the title will be automatically capitalized.</p>
+</div>
+```
+
+Optionally, you can use custom titles. For instance:
+
+```md
+!!! danger "Don't try this at home"
+    ...
+```
+
+will render:
+
+```html
+<div class="admonition danger">
+<p class="admonition-title">Don't try this at home</p>
+<p>...</p>
+</div>
+```
+
+If you don't want a title, use a blank string `""`:
+
+```md
+!!! important ""
+    This is a admonition box without a title.
+```
+
+results in:
+
+```html
+<div class="admonition important">
+<p>This is a admonition box without a title.</p>
+</div>
+```
+
+rST suggests the following `types`, but you're free to use whatever you want:
+    attention, caution, danger, error, hint, important, note, tip, warning.
+
+Styling
+-------
+
+There is no CSS included as part of this extension. Look up the default
+[Sphinx][sphinx] theme if you need inspiration.
+
+[sphinx]: http://sphinx.pocoo.org/