Build documentation with sphinx

[niccokunzmann]

In order for #204 to work, we need to build the documenation with Sphinx.

• split up the documentation into different sections
• build it with sphinx & the readthedocs theme

---

We're using Polar.sh so you can upvote and help fund this issue. We receive the funding once the issue is completed & confirmed by you. Thank you in advance for helping prioritize & fund our work.