Documentation Page for HGAM

[andreazanetti]

Adding a documentation page for HGAM.
The idea is to pass the following information to the user:

a) high level perspective of what HGAM is
b) role of BFS ordering of the graph batch for HGAM to work
c) support in the neighbor loader
d) practical working example of TrimToLayer class
e) pointers to external resources

This is a draft, asking for preliminary feedback.
Thanks for any input!

Contributors: @mszarma, @rBenke (robert.benke@intel.com), manos.farsarakis@intel.com

[rusty1s]

I think this looks good, thank you :) Eventually, we can extend this tutorial by some figures to show the difference in GNN computation visually as well.

[andreazanetti]

Thanks Matthias for working on it, and merging it!
We will then open a new PR in the next days, adding some figures to show pictorially how computation changes with HGAM (HSN).