Generate HTML documentation using MkDocs (WIP)

MkDocs is a static site generator geared towards building project documentation.
Documentation source files are written in Markdown, and configured with a single YAML file.

 * http://www.mkdocs.org/
 * http://www.mkdocs.org/user-guide/configuration/

Ref. #312

* remove pandoc-generated HTML documentation
* move markdown doc to doc/md/,
* mkdocs.yml:
  * generate HTML doc in doc/html
  * add pages TOC/ordering
  * use index.md as index page
* Makefile: remove execute permissions from generated files
* Makefile: rewrite htmlpages GFM to markdown conversion using sed:
   awk expression aslo matched '][' which causes invalid output on complex links with images or code blocks
* Add mkdocs.yml to .gitattributes, exclude this file from release archives
* Makefile: rename: htmldoc -> doc_html target
* run make doc: pull latest markdown documentation from wiki
* run make htmlpages: update html documentation

tpl/default/page.footer.html

@@ -1,3 +1,13 @@
+<<<<<<< HEAD
+=======
+<div id="footer">
+  <strong><a href="https://github.com/shaarli/Shaarli">Shaarli</a></strong>
+  - The personal, minimalist, super-fast, database free, bookmarking service by the Shaarli community
+  - <a href="doc/html/index.html" rel="nofollow">Help/documentation</a>
+    {loop="$plugins_footer.text"}
+        {$value}
+    {/loop}
+>>>>>>> Generate HTML documentation using MkDocs (WIP)
 </div>
 
 <div class="pure-g">