Add overview page for code gallery

[jpthiele]

As discussed in #7 the old overview website has not yet been ported.

Since @tjhei raised the valid point that the 'how to contribute' part should rather live in the code-gallery repo
I left that part out for now.

[tamiko]

@jpthiele Something went wrong for me. Instead of showing a radio button I see the following blurbb on the page:

[:octicons-image-16: Codes](https://dealii.org/developer/doxygen/deal.II/CodeGallery.html){ .md-button .md-button--primary .center }<br> See the list of programs that are part of the code gallery. { .card } [:octicons-download-16: Download](#downloading-the-code-gallery){ .md-button .md-button--primary .center }<br> Downloading the programs of the code gallery is easy. { .card } [:fontawesome-solid-paper-plane: Contribute](https://github.com/dealii/code-gallery){ .md-button .md-button--primary .center }<br> Contribute your own codes as a starting point for others. { .card } 

Also, the menu item for the code gallery points to the doxygen documentation instead of the new code_gallery.md. We need to fix that link as well: (in mkdocs.yml)

   42     - Code Gallery: https://dealii.org/developer/doxygen/deal.II/CodeGallery.html

[jpthiele]

@jpthiele Something went wrong for me. Instead of showing a radio button I see the following blurbb on the page:

[:octicons-image-16: Codes](https://dealii.org/developer/doxygen/deal.II/CodeGallery.html){ .md-button .md-button--primary .center }<br> See the list of programs that are part of the code gallery. { .card } [:octicons-download-16: Download](#downloading-the-code-gallery){ .md-button .md-button--primary .center }<br> Downloading the programs of the code gallery is easy. { .card } [:fontawesome-solid-paper-plane: Contribute](https://github.com/dealii/code-gallery){ .md-button .md-button--primary .center }<br> Contribute your own codes as a starting point for others. { .card } 

The <div> was on the same indentation level as the list, which lead to it not being rendered properly

Also, the menu item for the code gallery points to the doxygen documentation instead of the new code_gallery.md. We need to fix that link as well: (in mkdocs.yml)

   42     - Code Gallery: https://dealii.org/developer/doxygen/deal.II/CodeGallery.html

Done.

For some reason I was unable to force push a squashed commit, so it's two for now.