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, 86 insertions, 0 deletions

diff --git a/docs/extensions/index.md b/docs/extensions/index.md
new file mode 100644
index 0000000..34f8084
--- /dev/null
+++ b/docs/extensions/index.md
@@ -0,0 +1,86 @@
+title: Extensions
+
+Available Extensions
+====================
+
+Python Markdown offers a flexible extension mechanism, which makes it possible
+to change and/or extend the behavior of the parser without having to edit the
+actual source files.
+
+To use an extension, pass it to markdown with the `extensions` keyword.
+
+```python
+markdown.markdown(some_text, extensions=[MyExtension(), 'path.to.my.ext', 'markdown.extensions.footnotes'])
+```
+
+See the [Library Reference](../reference.md#extensions) for more details.
+
+From the command line, specify an extension with the `-x` option.
+
+```bash
+python -m markdown -x markdown.extensions.footnotes -x markdown.extensions.tables input.txt > output.html
+```
+
+See the [Command Line docs](../cli.md) or use the `--help` option for more details.
+
+!!! seealso "See Also"
+    If you would like to write your own extensions, see the
+    [Extension API](api.md) for details.
+
+Officially Supported Extensions
+-------------------------------
+
+The extensions listed below are included with (at least) the most recent release
+and are officially supported by Python-Markdown. Any documentation is
+maintained here and all bug reports should be made to the project. If you
+have a typical install of Python-Markdown, these extensions are already
+available to you using the "name" listed in the second column below.
+
+Extension                            | "Name"
+------------------------------------ | ---------------
+[Extra]                              | `markdown.extensions.extra`
+    [Abbreviations][]      | `markdown.extensions.abbr`
+    [Attribute Lists][]    | `markdown.extensions.attr_list`
+    [Definition Lists][]   | `markdown.extensions.def_list`
+    [Fenced Code Blocks][] | `markdown.extensions.fenced_code`
+    [Footnotes][]          | `markdown.extensions.footnotes`
+    [Tables][]             | `markdown.extensions.tables`
+    [Smart Strong][]       | `markdown.extensions.smart_strong`
+[Admonition][]                       | `markdown.extensions.admonition`
+[CodeHilite][]                       | `markdown.extensions.codehilite`
+[HeaderId]                           | `markdown.extensions.headerid`
+[Meta-Data]                          | `markdown.extensions.meta`
+[New Line to Break]                  | `markdown.extensions.nl2br`
+[Sane Lists]                         | `markdown.extensions.sane_lists`
+[SmartyPants]                        | `markdown.extensions.smarty`
+[Table of Contents]                  | `markdown.extensions.toc`
+[WikiLinks]                          | `markdown.extensions.wikilinks`
+
+[Extra]: extra.md
+[Abbreviations]: abbreviations.md
+[Attribute Lists]: attr_list.md
+[Definition Lists]: definition_lists.md
+[Fenced Code Blocks]: fenced_code_blocks.md
+[Footnotes]: footnotes.md
+[Tables]: tables.md
+[Smart Strong]: smart_strong.md
+[Admonition]: admonition.md
+[CodeHilite]: code_hilite.md
+[HeaderId]: header_id.md
+[Meta-Data]: meta_data.md
+[New Line to Break]: nl2br.md
+[Sane Lists]: sane_lists.md
+[SmartyPants]: smarty.md
+[Table of Contents]: toc.md
+[WikiLinks]: wikilinks.md
+
+Third Party Extensions
+----------------------
+
+Various individuals and/or organizations have developed extensions which they
+have made available to the public. A [list of third party extensions][list]
+is maintained on the wiki for your convenience. The Python-Markdown team
+offers no official support for these extensions. Please see the developer of
+each extension for support.
+
+[list]: https://github.com/Python-Markdown/markdown/wiki/Third-Party-Extensions