README rework

[jjbrosnan]

This PR reworks our README in the root directory of deephaven-core. It uses Matt's PR as a launch point. Some comments from that PR can't currently be applied:

• There are no docs for R. The only documentation comes from the ? operator in R.
• I don't link directly to the how to launch and build from source guide because it's out of date. I've made a PR to update that document, and plan on doing that soon. When that's done, I'll link to it directly.
• I couldn't think of a great way to link to some of our most important docs. I'll likely come up with a good solution soon.

[elijahpetty]

Not sure if a README is classified as a how-to guide or a reference doc, so I'm not sure if the tone matches the style guide. No significant comments, though.

[mattrunyon]

The info I had put in my PR is more contributing info and how to work in the codebase. This does trim down some of the main readme, but IMO there's still info that doesn't belong like monitoring docker logs or maybe even the PSK info.

Just my opinion

• README/GitHub is for developers who will be editing code in the repo. The start of the readme should kick users to the docs site if they just want to install and run Deephaven
• Docs site is for using Deephaven the product. Some developer info like creating plugins might belong there

Having user info in the readme just doubles maintenance. If they are out of sync it could also create some confusion. Is the repo readme the bleeding edge docs, or is it out of date?

Just for an example, look at the Tensorflow or ScikitLearn readmes. They have a blurb about what the repo is, a very basic install (in their case it's just a pip install line) and then link out to detailed instructions and tutorials. Then development/contribution info.

[devinrsmith]

I think we have a lot of issues w/ the related docs. Not necessarily something to tackle w/ this PR, but I think there's a lot of follow-up we should do.

I'm wondering if all the building and required dependency details should be moved to BUILD.md or DEVELOPMENT.md doc? We can link to it from here. I imagine the ideal readme would simply provide a single docker command to get up and running.

[mattrunyon]

I still think there is too much duplicated content between the readme and docs site. IMO there should be no duplicated content between the two and the readme should hard push users to the docs site where the start guides are maintained. I think any actual content here should be unique from the docs site. No reason to have duplicates of the guides which we'll forget to update again in this readme

[margaretkennedy]

Can we keep this README as bare bones as possible and point users to the docs site? The benefit of that is the maintenance burden falls on one page instead of two.