Add custom Sphinx extension that lists examples based on model name

[jessica-mitchell]

This PR adds a custom Sphinx extension that displays a list of examples based on model name.

How to use it
Add directive to reStructured Text with required argument - the model_name:

.. listexamples:: model_name

This was used in models/iaf_psc_alpha.h and aeif_cond_alpha
See output here

Updated

• Script now includes function that finds matches of model names in examples
• The titles of examples are displayed rather than filename
• Using the ref role syntax, which should allow the use of this directive anywhere in the documentation without issue (hopefully)

TODO

• Rename some example titles that are unclear
• Fix sorting so it's used on the titles rather than filenames

[pnbabu]

I think sorting the list of files by title text rather than the filenames is more useful. I see that it's listed as a TODO in this PR. Is that planned to be addressed here?

[jessica-mitchell]

@pnbabu I finally managed to sort the examples by title; this required a fair bit of reworking the extension script so its quite different now

[pnbabu]

Looks good to me! This list is super helpful for someone browsing through the models. I think we could add the directive to all model files once you have approval from other reviewers as well.

[terhorstd]

Two bytes to cut, then I think it's ready to go.
Thanks for the cool upgrade! 👍