First pass at making "Getting Started" guide better

[brookslogan]

Currently, "Getting Started" has installation instructions + a bunch of stuff on getting things into epi_df format + some plotting + nothing on other features of the package.

Suggestion:

• Migrate the epi_df stuff to another vignette or combine it with another vignette.
• Have Getting Started be installation instructions + pointers to various function reference pages and/or vignettes / the tooling book.

Or is there a way to just eliminate the Getting Started guide and just have the landing page serve that purpose?

[nmdefries]

Open-ended

• Landing page = installation + very basic usage
• Getting started = medium-complexity use? quick start guide? Description of what you might want to do with links to relevant vignettes?

[dshemetov]

• Attributions on the Getting Started page are a little awkward
• No epi_archive examples
• No discussion of the benefits of using an epi_df, would like to see a plot with epi_df
• "Some details on metadata" and "Using additional key columns in epi_df" could likely move to another reference page

[dsweber2]

Can we demo "easier" data sets than Covid cases (where forecasting is more tractable) or provide better context about performance

From a bullet point on an OKR, probably a good way to make this concrete