Add a section in installation docs about running NEST with EBRAINS

[jessica-mitchell]

This PR, as discussed in docathon with @clinssen @steffengraber @heplesser @ddahmen , adds the try it on ebrains button for an installation free option in the install section.

Note that the links directly takes the person to a one neuron example (assuming they are logged and have server running). The reason: all the examples will be loaded, and the kernel that contains NEST should be set. So less changes for user. We would need to add additional instructions if we want to open a plain notebook on EBRAINS, which is another possibility.

Missing from the instructions so far (wip in another branch, but can be added here) is this issues that you have to basically click twice to load an example if you dont have a server running yet.

#3066 #3067 should be merged first!

See output for Read the Docs here