Fix js and favicon

See merge request !4

* Add version number to override old cached favicons in browsers

Create mobile menu

Closes #5

See merge request !2

* Hide expanded dropdown menu, when window is resized and device width
is wider than mobile breakpoint view.

Fix footer with Gitlab and license links #4

Closes #4

See merge request !3

* Footer-headline is a full-width row now
* Links are seperated in two columns
* The license link is less prominent than all other links and gets a
lighter color

* add new column with github and license links
* replace menu item „Code“ with „Source“

Update onepager layout and content

* Issue #1: Add demo link to banner section
* Issue #2: Replace icons with gifs and reorder layout in detail section 
* Issue #3: Add section with linked showcase previews

@wolfgang  This PR is ready for review and merge.

![TEI-Publisher-neue-hp](/uploads/fa04b3ef26a9b7dfbb124a44268f1b4a/TEI-Publisher-neue-hp.png)

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@wolfgang @joern 
**Please note**
* Please exchange the animated gifs and compress as much as possible.  
The current size of the page is 4.6MB, far too much, just because of 4.3MB gif size!
* Please also have a look, if you can find some other images for the showcases in the last section:  
They have a poor quality and differ in size.
* The links I added to the gifs lead to chapters in the doc. Please check, if that makes sense, especially if you consider to update the doc content soon.
* The site needs more content -> text. Be careful before deploying the page - there are still some "lorem ipsum" placeholders in the markup!

See merge request !1

* add demo section (overview)
* link images with doc url
* add showcase section
* add styling