doc: documentation rework

[dshemetov]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

• Turned the README.md into a README.Rmd and rewrote it
  • Resolves Comment RNG reproducibility wrapper on "Get started" page #364
  • Resolves Fix RNG reproducibility wrapper around non-RNG quantity #270
• Rewrote the Getting Started
  • Resolves First pass at making "Getting Started" guide better #350
• Consolidated aggregation and slide into the new epi_df vignette, rewrote them both
  • Resolves Consider including time series completion code in vignettes  #239
  • Resolves Should include comparison with slider functions #427
  • Resolves "Slide" vignette reference to and conclusions about Texas reporting need to be expanded #367
  • Resolves epiprocess vignette "other key" example should use county as geo_value and state as additional key  #152
• Rewrite epi_archive vignette
  • Resolves Consider improving nowcaster/forecaster epi_slide sample in advanced.Rmd #288
  • Resolves Move prob_arx() in advanced vignette over to epipredict #54
• Minor naming tweaks to other vignettes
  • Resolves Use autoplot where appropriate in vignettes #421
• Pushed the Compactify vignette into the More Articles... section
  • Resolves [DOC] Centralize and clarify documentation on mutation, compactification, and other footguns #444
• Re-organized and consolidated the reference pages in _pkgdown.yml; gave them a custom ordering to better prioritize functions of interest
• Consolidated a few functions into single reference pages (such as epi_slide_opt and the detect_outlier pages)
• Added a pkgdown workflow utility
• Updated slide reference pages
  • Resolves Describe accepted return types/classes of f in epi[x]_slide roxygen #234
  • Resolves Clean up epi_slide f parameter documentation  #143
  • Resolves Add function or prominent documentation about how to get a single group&reftime slide input  #60
• Updated epix_fill_through_version docs
  • Resolves Doc fixes #419
• Sped up compilation of vignettes by adding cache=TRUE as a default knitr option, to cache all vignette code chunks
  • Resolves Make vignette building faster #363
  • Resolves Figure out why some eval=FALSE vignette chunks are still (partially?) running #190
• Copied .editorconfig from epidatr here
  • Resolves Consider adding dev editor config, setup guide, styler, styler hooks #226
• Used usethis::use_package_doc() to migrate epiprocess.R to epiprocess-package.R and re-organized that file a bit
  • Resolves Make "package" type help work #353
• Documented epi_df decaying to tibble more prominently
  • Resolves select.epi_df falls back to tibble if geo_value or time_value are removed #505
• Small tweak to as_epi_df code
  • Resolves This isn't the right way to do this #308

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