Restructure installation documentation

[RudolfWeeber]

The high level of detail, eventhough, necessary, might make the installation instrucitons in the user guid a bit inaccessible.

We can probably improve that by restructuring a bit:
Mainly, we should aim for a better separation between the basic (necessary) steps on the one hand, and detailed explanations and advanced topics, on the other hand.

At the top, maybe we can add a super-short overview with links: Installing ESPResso usually involves the follwoing steps

• installing depenencies [Ubuntu] [macOS] [other platforms] [explanation of dependnecies]
• downloading the source code [release version] [develoment version]
• configuring and compiling

Also put the link to the installation FAQ there.

For CUDA, mention, that user's should only bother, if they intend to use one of the GPU algorithms.

For the dependency installation seciont (particularly Ubuntu),
Make a more clear distinction what are "standard" and what is (optional) more advanced stuff such as Jupyter.