Update configuration of mkdocs documentation

This configures the mkdocs documentation to use the `mkdocs-material` theme
and sets the colors to orange and blue.

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## Not released yet
 
-- 2022-05-17: Added base setup for extended documentation to be hosted on ReadTheDocs and being created by `mkdocs`.
+- 2022-05-17: Added base setup for extended documentation to be hosted on ReadTheDocs and being created by `mkdocs`. Also adds a base configuration and style of the documentation.
 - 2022-05-17: Added configuation for Markdown linter `markdownlint` and added it to CI. The linter can be called locally by typing `mdl .` from the root of the repository. This also led to an updated configuration of the CI.
 - 2022-05-17: Add some more documentation on how to install the adapter to the `README.md`.
 - 2022-02-18: Updated CI to use images from account `ajaust` from Dockerhub. Changed tolerance for partitioned tests to 5e-5 due to minimal changes in the solution with the new images on a new VM.

doc/user/docs/index.md

-# Welcome to MkDocs
+# DuMuX-preCICE adapter documentation
 
-For full documentation visit [mkdocs.org](https://www.mkdocs.org).
-
-## Commands
-
-- `mkdocs new [dir-name]` - Create a new project.
-- `mkdocs serve` - Start the live-reloading docs server.
-- `mkdocs build` - Build the documentation site.
-- `mkdocs -h` - Print help message and exit.
-
-## Project layout
-
-```text
-mkdocs.yml    # The configuration file.
-docs/
-    index.md  # The documentation homepage.
-    ...       # Other markdown pages, images and other files.
-```
+!!! info
+    This documentation homepage is under construction.
\ No newline at end of file

doc/user/mkdocs.yml

 site_name: DuMuX-preCICE adapter
+
+theme:
+  name: 'material'
+  palette:
+    primary: deep orange
+    accent: blue
+
+repo_name: dumux-appl/dumux-preCICE
+repo_url: https://git.iws.uni-stuttgart.de/dumux-appl/dumux-precice
+
+markdown_extensions:
+  - admonition
+