Add wiki pages to the libGDX website

[crykn]

This PR ports the libGDX wiki to the libGDX website. The main reasoning behind this is that GitHub wikis are no longer indexed by search engines, making the wiki far less useful.

Closes #58. Closes #62. See Status Report #8.

How does it work?

The old wiki markdown files were ported to Jekyll with the help of some code by @Spaio. Thank you very much for this!

• The main step involved was fixing the wiki-style links ([[streaming-music]] -> [streaming-music](wiki/audio/streaming-music.md)) and, in the course of this, organising the pages in subdirectories to make them more manageable.
• Similarly, the paths of assets located in the old wiki repo were changed to refer to their new location in /assets/wiki/.
• In addition, every wiki page now needs a frontmatter specifying a page title:

  ---
  title: Modules overview
  ---

• The wiki pages are now available via the /wiki/ subsection of the website which uses a modified version of the Minimal Mistakes theme; see _sass/wiki.scss; _layouts/default_wiki.html, _layouts/wiki.html; the ToCs can be found at _includes/wiki_index.md and _includes/wiki_sidebar.md
• Previous progress on this project can be seen here. The old wiki is still accessible via the history.

For anyone stumbling upon this PR later on, since then we've done a couple of changes to the files seen in this PR. We've also recently introduced a simple search feature for wiki pages (wiki/search.md, _includes/wiki_masthead.html) which uses Lunr to index the full page content of the wiki pages, and we added a custom "copy to clipboard" action (assets/js/clipboard.js; added to the after_footer_scripts; the unminified code can be found here).

Future Ideas

• Embedding GWT content (Embed GWT apps #68), which would allow us to include actual executable libGDX examples directly in the wiki: the JS code should be located somewhere in /assets/wiki; then a <div> and the corresponding script (see the index.html generated by a libGDX GWT project) needs to be added to the wiki page; the GWT application itself requires some (asset) path changes. Later on, a custom embed-gwt element (see here for a very basic example of an element) should be created to simplify usage.