Add initial setup for readthedocs documentation

This adds:
- a freshly initialized mkdocs configuration in `doc/user`
- a ReadTheDocs configuration in the root of the repository
  named `.readthedocs.yaml`

.readthedocs.yaml

+version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+
+mkdocs:
+  configuration: doc/user/mkdocs.yml
+  fail_on_warning: false
+
+python:
+   install:
+   - requirements: doc/user/requirements.txt
\ No newline at end of file

CHANGELOG.md

@@ -2,6 +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 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
+
+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.
+```

doc/user/mkdocs.yml

+site_name: DuMuX-preCICE adapter

doc/user/requirements.txt

+mkdocs-material
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