Add pkgdown and build docs

[chinandrew]

Here it is hosted on my account: https://chinandrew.github.io/quantgen/

Summary of Changes:

• Add pkgdown yaml
• Move notebooks to vignettes folder and add vignette metadat that I copy pasted from covidcast vignettes. NOTE: I moved and didn't build two vignettes to temp_vignette_storage/ just because they took forever to build. They'll need to be moved and rebuilt whenever we're ready to go live.
• Added a filler "Get started" page with no info
• Added section numbers to vignettes (order was arbitrary) just to follow the covidcast R package convention.
• Build documentation

To activate the site after merging: Go to repo settings, scroll down to Github pages, and set the branch to main and the folder to docs/.

To rebuild: Rerun pkgdown::build_site(), which will build and deposit all the necessary files into the docs/ folder

Additional pkgdown example on the R client's pkgdown file

[ryantibs]

Thank you so much @chinandrew. Much appreciated!