Add overview page for code gallery

docs/gallery_applications/applications.md

@@ -18,4 +18,4 @@ There are many outstanding applications that make use of deal.II. This is just a
 - [preCICE](https://www.precice.org)
 - [PRISMS](http://www.prisms-center.org/#/ctools/software)
 
-Also see [Code Gallery](https://www.dealii.org/code-gallery.html) for smaller example programs using deal.II.
+Also see [Code Gallery](code_gallery.md) for smaller example programs using deal.II.

docs/gallery_applications/code_gallery.md

@@ -0,0 +1,60 @@
+Code Gallery
+=============
+
+The deal.II "code gallery" is an extension to the deal.II tutorial. 
+While the tutorial's emphasis is on teaching aspects of deal.II by providing concrete examples — 
+and extensive documentation and discussion — 
+of how parts of deal.II are used in applications, 
+the intent of the code gallery is to *provide starting points* for your own codes 
+by making available codes others have already written. 
+
+As a general rule:
+
+* The code gallery is not part of deal.II. 
+  The applications in the code gallery were written by users 
+  who wanted to make their codes available
+* The applications in the code gallery are generally 
+  not as well tested or documented as the tutorial programs.
+  **These codes come with no warranty at all**.
+  If you have questions, you need to contact the authors of these codes, not the authors of deal.II
+* **We welcome additions and improvements to existing codes, 
+  as well as new code gallery applications!** 
+  Most users start their own applications from an existing tutorial program.
+  Having more starting points helps others, 
+  but it also gives code gallery authors additional ways to get recognition for their own codes! 
+  See [the code gallery repository](https://github.com/dealii/code-gallery) for instructions on how to contribute.
+
+
+  <div class="grid" markdown>
+
+  [: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 }
+
+
+  </div>
+
+
+Downloading the code gallery
+----------------------------
+
+You can get the entire code gallery from its GitHub home by using the command 
+
+```
+git clone https://github.com/dealii/code-gallery.git
+```
+
+This creates a directory `code-gallery/` that contains all gallery programs.
+
+If you want to generate the deal.II documentation locally
+and want it also to cross-link to the code galery, you need to do the step above withing your copy of deal.II, 
+i.e., so that the command above creates a directory `code-gallery/`
+parallel to the `include/`, `source/` and `/examples` (and other) directories.

docs/gallery_applications/index.md

@@ -2,5 +2,5 @@ Gallery and Applications
 ========================
 
 - [Images and Videos](gallery.md)
-- [Code Gallery](https://dealii.org/developer/doxygen/deal.II/CodeGallery.html) - examples programs using deal.II
+- [Code Gallery](code_gallery.md) - examples programs using deal.II
 - Collection of [Applications using deal.II](applications.md)