Add detailed neuron documentation

[jessica-mitchell]

This PR adds 2 new pages detailing neurons in NEST for users to better understand what's available in NEST from a comp neuroscience perspective.

the first page describes the various neuron types

• new tags were created for models to align with the neuron types

the second page describes the update algorithm for neurons using iaf_psc_alpha as the example.
We use this model as an example as it's one that has extensive documentation updates, and is used as an example in many cases.
Further flowcharts will be considered in the future to describe other model types like rate neurons, but that is for another PR.

This PR focuses on neurons - but there are plans to add synase docs in another PR

See the output here: https://nest-simulator--3448.org.readthedocs.build/en/3448/neurons/neuron_types.html
https://nest-simulator--3448.org.readthedocs.build/en/3448/neurons/neuron_update.html

[tomtetzlaff]

Very nice. Thanks a lot @jessica-mitchell. I added a few minor comments on the "Types of neurons" page.

[ddahmen]

Thanks a lot Jessica for the great work! I only found one additional typo. And the flowchart has two rendering problems: 1. the "Delta t" in the "handle input spikes box" is broken. 2. The numerator of the ratio in the refractoriness box is cut off.
After fixing these two problems, the PR is ready to be merged. Thanks!