🔧 Add a file with direct dependencies

The requirements.in` file contains only the direct project dependencies.
I suggest to use it from now on to pin the top-level project dependencies,
instead of `requirements.txt` and use it to generate requirements.txt
like so:

python -m pip install -r requirements.in
python m pip freeze    > requirements.txt

Author: ebouchut

requirements.in

@@ -0,0 +1,7 @@
+Jinja2==3.1.5
+mkdocs==1.6.1
+mkdocs-htmlproofer-plugin==1.3.0
+mkdocs-material==9.5.49
+mkdocs-redirects==1.2.2
+mkdocs-unused-files==0.2.0
+pymdown-extensions==10.13