docs: add documentation on how to test the website

Note: since hugo does not support GitLab's math extension, these needs to be converted to some other format that is not using

backticks, i.e. [pandoc's tex_math_dollars format](https://pandoc.org/MANUAL.html#extension-tex_math_dollars). However, this

also means that explicit backticks (like `` $` `` or `` ```math ``) can't be used (e.g. to explain the GitLab's math mode).

## Testing the website

You can easily test the website of the documentation with your changes yourself by running:

```sh

git clone --recurse-submodules https://gitlab.cern.ch/allpix-squared/allpix-squared-website.git

APSQ_REPO=your_user_name/allpix-squared APSQ_REF=branch_name ./get_artifacts.sh

hugo server

```

For more information refer to [the README](https://gitlab.cern.ch/allpix-squared/allpix-squared-website#commands) in the

website repository.