Big docs & readme update

[barucden]

Okay, this one's a little bigger.

The goal is to simplify the docs: to contain only what's this package about so we can keep it up-to-date, no duplicates (rather use cross-references), simpler structure, less talking.

The biggest change in README is the addition of contributing section.

Main changes in docs:

• Menu structure is simpler
• New example for integration with Flux.jl
• Added docs for MapFun and AggregateThenMapFun
• Removed the "Working with images in Julia" page (instead, it is only a tiny section in introduction that refers to JuliaImages docs)
• Removed indices.md

I plan to do a few more smaller changes in the future (like adding more examples, simplify further, etc.)

[barucden]

Yeah, the sentences read nicely. For example, the part about images in Julia was quite educational. I almost felt pity removing it. On the other hand, maintaining the documentation is perhaps harder than maintaining the code, so I aimed to cut down anything that is not necessary (in this specific case, the documentation is available elsewhere -- JuliaImages docs). Also, I think having less text is better for the users as it's easier to skim through.

Edit: Let's keep this PR open for a while so @Evizero has a chance to express his opinion.

[Evizero]

Thanks for the kind words. While it was fun writing the images tutorial page, I do think it became outdated quite a while ago. I defer to your combined judgement which has been excellent so far. Thank you for your hard work

[barucden]

Thanks for the feedback! I am merging it in now. We can continue to improve the documentation iteratively.