Feature/new documentation

[c-moeller]

Fix #Issue

This is the new documentation for the open_plan tool. It is based on the MVS documentation.

This PR is work in progress.

Changes proposed in this pull request:

• Your_changes

The following steps were realized, as well (if applies):

• Apply black (black . --exclude docs/)
• Update the CHANGELOG.md

The following optional steps were realized:

• Use in-line comments to explain your code
• Write docstrings to your code
• For new functionalities: explain in readthedocs
• Write test(s) for your new patch of code

_{For more information on how to contribute check the [CONTRIBUTING.md](https://github.com/rl
-institut/open_plan/blob/dev/CONTRIBUTING.md).}

[Bachibouzouk]

@FredericHirschmueller - since you deleted the file the docs will not compile (indicated by the little red cross next to the commit and by "All checks have failed"). It is not a big deal, I guess it will work again once we add the file. Now I activated build of the docs when you commit changes to this branch: look in the checks at the bottom of the PR and click on the docs link (provided the build did not fail ;)). Did you just modified text without compiling the docs locally?