Add link to 3.7 whats new page in docs

[jessica-mitchell]

Make 3.7 link available for release

[jessica-mitchell]

@heplesser @terhorstd Are there any other key changes we want to inform users about in the what's new section for 3.7?
Here is what we have so far: https://nest-simulator--3153.org.readthedocs.build/en/3153/whats_new/v3.7/index.html#release-3-7

[heplesser]

@jessica-mitchell I would suggest to add the following new models/features

• Add iaf_tum_2000: a neuron model with integrated short-term plasticity synapse #2971
• Add ignore_and_fire neuron model #2987
• New connection rule for Poisson-distributed number of synapses between neuron pairs #2960

Also, once merged, we should mention the new interface for extension modules, this is a breaking change:

• Create NEST Extension Interface for NEST compatible with new registration scheme and unloading #3103
• Move registration of ConnBuilders, Backends, growth curves from nestmodule to respective managers, allowing unloading #3149

[jessica-mitchell]

Also, once merged, we should mention the new interface for extension modules, this is a breaking change:

* [Create NEST Extension Interface for NEST compatible with new registration scheme and unloading #3103](https://github.com/nest/nest-simulator/pull/3103)

* [Move registration of ConnBuilders, Backends, growth curves from nestmodule to respective managers, allowing unloading #3149](https://github.com/nest/nest-simulator/pull/3149)

I see changes to code but nothing really to the documentation for the nest-extension module - what does the user need to know about these changes? What does the user need to do differently?

[jessica-mitchell]

@heplesser ping!

[jessica-mitchell]

Merging as discussed at hackathon