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, 87 insertions, 0 deletions

diff --git a/docs/extensions/footnotes.md b/docs/extensions/footnotes.md
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+title: Footnotes Extension
+
+Footnotes
+=========
+
+Summary
+-------
+
+The Footnotes extension adds syntax for defining footnotes in Markdown
+documents.
+
+This extension is included in the standard Markdown library.
+
+Syntax
+------
+
+Python-Markdown's Footnote syntax follows the generally accepted syntax of the
+Markdown community at large and almost exactly matches [PHP Markdown Extra][]'s
+implementation of footnotes. The only differences involve a few subtleties in
+the output.
+
+[PHP Markdown Extra]: http://michelf.com/projects/php-markdown/extra/#footnotes
+
+Example:
+
+```md
+Footnotes[^1] have a label[^@#$%] and the footnote's content.
+
+[^1]: This is a footnote content.
+[^@#$%]: A footnote on the label: "@#$%".
+```
+
+A footnote label must start with a caret `^` and may contain any inline text
+(including spaces) between a set of square brackets `[]`. Only the first
+caret has any special meaning.
+
+A footnote content must start with the label followed by a colon and at least
+one space. The label used to define the content must exactly match the label used
+in the body (including capitalization and white space). The content would then
+follow the label either on the same line or on the next line. The content may
+contain multiple lines, paragraphs, code blocks, blockquotes and most any other
+markdown syntax. The additional lines must be indented one level (four spaces or
+one tab).
+
+When working with multiple blocks, it may be helpful to start the content on a
+separate line from the label which defines the content. This way the entire block
+is indented consistently and any errors are more easily discernible by the author.
+
+```md
+[^1]:
+    The first paragraph of the definition.
+
+    Paragraph two of the definition.
+
+    > A blockquote with
+    > multiple lines.
+
+        a code block
+
+    A final paragraph.
+```
+
+Usage
+-----
+
+See [Extensions](index.md) for general extension usage, specify
+`markdown.extensions.footnotes` as the name of the extension.
+
+See the [Library Reference](../reference.md#extensions) for information about
+configuring extensions.
+
+The following options are provided to configure the output:
+
+* **`PLACE_MARKER`**:
+    A text string used to mark the position where the footnotes are rendered.
+    Defaults to `///Footnotes Go Here///`.
+
+    If the place marker text is not found in the document, the footnote
+    definitions are placed at the end of the resulting HTML document.
+
+* **`UNIQUE_IDS`**:
+    Whether to avoid collisions across multiple calls to `reset()`. Defaults to
+    `False`.
+
+* **`BACKLINK_TEXT`**:
+    The text string that links from the footnote definition back to the position
+    in the document. Defaults to `↩`.