Improve docs for guides and getting started

[jessica-mitchell]

This PR builds on #3066 focusing on the organization of the guides and tutorials - here the various 'guides' are organized with an initial section for beginners continuing to various topics that would be helpful for the majority of users who are relatively new to NEST. Additional guides are included after.
Can only be merged once #3066 is merged

[jessica-mitchell]

Please check the 'Next steps' section as I tried to put the topics logically together in some cases. I was trying to avoid having too many cards.

@clinssen @ddahmen @heplesser @steffengraber

See output here

Changes made

• have 3 sections
• CHANGE next steps: Create you own network model
• create models --> create neuron, synapse, devices

Move the following to 'next steps'

• random numbers

• stimulate the network

• spatially structured

• guide to parallel computing

• remove see also

• add examples card in next steps

• large scale models (brunel, microcircuit, meso, mam)
  NOTE: put large scale models in 'next steps', unsure if that was where we discussed it should go / also not sure about Brunel - I added one example from nest

• built in timers --> go to hpc

• nest sonata --> connect to other tools

• Changed button on index page that links to this page to "start exploring nest" rather than 'get started' since it focuses on the topcis of nest where as 'getting started' kind of implies that it includes installation

• added new icons

• remove unused icons

TODO
add an example from spatially structured networks to large scale models

[heplesser]

@jessica-mitchell This looks very good now! The "Large networks" box is a bit tall, but I don't see any way around that.

[ddahmen]

@jessica-mitchell Great work! Apart from the typo "how random number" -> "how random numbers" in the "Simulate your network" box, everything looks perfect to me.