Docs: improve copy-vs-view, mutability, and advanced API

[timholy]

@johnnychen94 is pioneering a move of StructArrays into JuliaImages and I was taking a look at the package. The documentation is quite good, but (as usual) can be even better, and so I thought I'd leverage my "newbie" status to do some good for the community.

This revision attempts to address the following perceived issues:

• the current documentation doesn't seem to mention that different methods of creating a StructArray differ in whether they create a view or a copy
• the issue above is conflated with mutability of the underlying structure. These are actually two separate issues and in my opinion should be disentangled.
• the advanced API section could be more explicit about how the specializations modify the behavior of the created StructArray.

The advanced API section might need further revisions; I think the portion of the demo involving Location might simply be a bug in StructArrays, but I haven't delved deeply. In any case, meta_table(s) throws an error so the demo doesn't work.

This also uses doctests more heavily to make sure the demos actually work.

Would appreciate a review from @piever and/or @jlchan if possible.

[timholy]

Given changes in printing over time, I'd recommend we disable doctesting on Julia 1.3. Happy to make this change if others approve.(did it)

[jlchan]

Thanks for the PR @timholy! I really like the new docs on modifying struct fields, and the additional documentation of AOS storage is helpful. I just added two small suggestions.

[johnnychen94]

I didn't realize that there was a docs when I created #199. Now the README and the docs are out of sync, I think we need to delete the README doc contents and leave a link to the hosted documentation site?

[timholy]

I think we need to delete the README doc contents and leave a link to the hosted documentation site?

Good idea. I can try to add your point to these docs too.

[timholy]

OK, I introduced the dims=... constructor in the introduction. I struggled to describe the "counterintuitive" behavior for reinterpret (i.e., #197) because to me it's not counterintuitive, so I left that part out. I also emptied the README and redirected readers to the stable documentation by default. (Developers should know how to access the dev documentation from the pulldown.)