Reduce `ghcide` README and move some content to docs

It's nice for `ghcide` to have a small READMe since it's published on Hackage, but it's quite out of date and talks about a lot of things (e.g. feature support) that is already in the HLS docs and we only want to write once. So we should cut it down, move anything that seems useful to the main docs, and leave something relatively minimal.