Documentation improvements

It would be greatly appreciated if anyone wants to go over the documentation with fresh eyes (that's mainly why I added a "help wanted" label here), but a few things jump out even at me:

• The quick start example on the home page doesn't really showcase capabilities besides the most basic Cell workflow. I do think that's good for getting started quickly, but even as far as geometry-level layout we want to encourage users to place things in CoordinateSystems and render to Cell at the end. Keep in mind that this might be the user's first encounter with DeviceLayout.jl. Done -- the "hello world" example still uses Cell, but the First Layout tutorial introduces CoordinateSystems.
  • We should at least include the list of features from the README and link to the examples with pretty pictures. Done.
• Should note somewhere that these are general 2D/2.5D device CAD capabilities—we are focusing on the quantum use case right now since that's what we can most readily make examples and do support/outreach for
• Several pages mix narrative explanation and how-tos with interspersed @docs block references. It might be better to separate these (consider the four-document model). Done.
• Several pages have little or no explanation, just the reference. Reference and explanation are split.
• Troubleshooting/FAQ pages for geometry and schematic layout should be consolidated (can still use separate sections on one page)—users shouldn't have to think about whether to check one or the other. Done.
• Could use more how-tos and examples in general
• There are likely many minor issues like center(::Schematic, ::ComponentNode) appearing under center on the geometry page (rather than only on the page about schematics), or center(::Polygon) having its own docstring despite being the same as the generic center. (Those in particular are fixed by https://github.com/aws-cqc/DeviceLayout.jl/pull/58.)

See also #1, #2, #3, #14. I'm collecting these notes in one issue to help sort out the structural goals for major revisions, but we can break this down into multiple sub-issues/PRs.