Add custom model example to docs.

[Gregliest]

This PR adds a simple custom model example to the docs. As discussed in Slack, this example would have helped me understand how all the pieces fit together. But, happy to edit/move/delete as desired.

cc @ToucheSir

PR Checklist

• Tests are added
• Entry in NEWS.md
• Documentation, if applicable
• API changes require approval from a committer (different from the author, if applicable)

[DhairyaLGandhi]

We should link to other places in the docs that show similar patterns and help new user experiences. Like https://fluxml.ai/tutorials/2020/09/15/deep-learning-flux.html

[Gregliest]

Alright, I addressed all your comments. Looks like doctests are failing in basics.md, do you want me to have a look at that as well?

[ToucheSir]

Sorry about the delay. If you're still interested, I think fixing basics.md in a separate PR would be great (to my knowledge, it should be a simple matter of formatting). This is good to go though.

[logankilpatrick]

Can we bors this @ToucheSir ?

[DhairyaLGandhi]

I'm wondering if this makes sense as a tutorial in the model zoo. Showing not just the structure based model, but also one that doesn't involve any structure and is very functional. We already have a similar example in the vae model and the cppn etc, but I wonder if an even more basic one is warranted.

[logankilpatrick]

I am personally in favor of a heavier docs page vs model zoo. Currently, it's too much effort to have to go through the model zoo and figure out that you want. Having these in the docs makes it just a bit easier to find

[ToucheSir]

@logankilpatrick Bors is failing to integrate because of failures on the staging branch and possibly master, see discussion in #1797.

[ToucheSir]

bors r+

[bors]

Build succeeded:

• buildkite/flux-dot-jl