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, 0 insertions, 147 deletions

diff --git a/docs/extensions/extra.txt b/docs/extensions/extra.txt
deleted file mode 100644
index 5347d81..0000000
--- a/docs/extensions/extra.txt
+++ /dev/null
@@ -1,147 +0,0 @@
-title:      Extra Extension
-prev_title: Extensions
-prev_url:   index.html
-next_title: Abbreviations Extension
-next_url:   abbreviations.html
-
-Python-Markdown Extra
-=====================
-
-Summary
--------
-
-A compilation of various Python-Markdown extensions that (mostly) imitates
-[PHP Markdown Extra](http://michelf.com/projects/php-markdown/extra/).
-
-The supported extensions include:
-
-* [Abbreviations](abbreviations.html)
-* [Attribute Lists](attr_list.html)
-* [Definition Lists](definition_lists.html)
-* [Fenced Code Blocks](fenced_code_blocks.html)
-* [Footnotes](footnotes.html)
-* [Tables](tables.html)
-* [Smart Strong](smart_strong.html)
-
-See each individual extension for syntax documentation. Extra and all its 
-supported extensions are included in the standard Markdown library.
-
-Usage
------
-
-From the Python interpreter:
-
-    >>> import markdown
-    >>> html = markdown.markdown(text, ['markdown.extensions.extra'])
-
-There may be [additional extensions](index.html) that are distributed with
-Python-Markdown that are not included here in Extra. The features 
-of those extensions are not part of PHP Markdown Extra, and 
-therefore, not part of Python-Markdown Extra. If you really would 
-like Extra to include additional extensions, we suggest creating 
-your own clone of Extra under a different name 
-(see the [Extension API](api.html)).  
-
-Markdown Inside HTML Blocks
----------------------------
-
-Unlike the other Extra features, this feature is built into the markdown core and
-is turned on when `markdown.extensions.extra` is enabled.
-
-The content of any raw HTML block element can be Markdown-formatted simply by
-adding a `markdown` attribute to the opening tag. The markdown attribute will be
-stripped from the output, but all other attributes will be preserved.
-
-If the markdown value is set to `1` (recommended) or any value other than `span`
-or `block`, the default behavior will be executed: `p`,`h[1-6]`,`li`,`dd`,`dt`,
-`td`,`th`,`legend`, and `address` elements skip block parsing while others do not.
-If the default is overridden by a value of `span`, *block parsing will be skipped*
-regardless of tag. If the default is overridden by a value of `block`,
-*block parsing will occur* regardless of tag.
-
-#### Simple Example:
-```
-This is *true* markdown text.
-
-<div markdown="1">
-This is *true* markdown text.
-</div>
-```
-#### Result:
-```
-<p>This is <em>true</em> markdown text.</p>
-<div>
-<p>This is <em>true</em> markdown text.</p>
-</div>
-```
-
-### Nested Markdown Inside HTML Blocks
-Nested elements are more sensitive and must be used cautiously. To avoid
-unexpected results:
-
-* Only nest elements within block mode elements.
-* Follow the closing tag of inner elements with a blank line.
-* Only have one level of nesting.
-
-#### Complex Example:
-```
-<div markdown="1" name="Example">
-
-The text of the `Example` element.
-
-<div markdown="1" name="DefaultBlockMode">
-This text gets wrapped in `p` tags.
-</div>
-
-The tail of the `DefaultBlockMode` subelement.
-
-<p markdown="1" name="DefaultSpanMode">
-This text *is not* wrapped in additional `p` tags.
-</p>
-
-The tail of the `DefaultSpanMode` subelement.
-
-<div markdown="span" name="SpanModeOverride">
-This `div` block is not wrapped in paragraph tags.
-Note: Subelements are not required to have tail text.
-</div>
-
-<p markdown="block" name="BlockModeOverride">
-This `p` block *is* foolishly wrapped in further paragraph tags.
-</p>
-
-The tail of the `BlockModeOverride` subelement.
-
-<div name="RawHtml">
-Raw HTML blocks may also be nested.
-</div>
-
-</div>
-
-This text is after the markdown in HTML.
-```
-#### Result:
-```
-<div name="Example">
-<p>The text of the <code>Example</code> element.</p>
-<div name="DefaultBlockMode">
-<p>This text gets wrapped in <code>p</code> tags.</p>
-</div>
-<p>The tail of the <code>DefaultBlockMode</code> subelement.</p>
-<p name="DefaultSpanMode">
-This text <em>is not</em> wrapped in additional <code>p</code> tags.</p>
-<p>The tail of the <code>DefaultSpanMode</code> subelement.</p>
-<div name="SpanModeOverride">
-This <code>div</code> block is not wrapped in paragraph tags.
-Note: Subelements are not required to have tail text.</div>
-<p name="BlockModeOverride">
-<p>This <code>p</code> block <em>is</em> foolishly wrapped in further paragraph tags.</p>
-</p>
-<p>The tail of the <code>BlockModeOverride</code> subelement.</p>
-<div name="RawHtml">
-Raw HTML blocks may also be nested.
-</div>
-
-</div>
-<p>This text is after the markdown in HTML.</p>
-```