Docs overhaul

[dsweber2]

Checklist

Please:

• Make sure this PR is against "dev", not "main".
• Request a review from one of the current epipredict main reviewers:
  dajmcdon.
• Make sure to bump the version number in DESCRIPTION and NEWS.md.
  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
  0.7.2, then write your changes under the 0.8 heading).
• Consider pinning the epiprocess version in the DESCRIPTION file if
  • You anticipate breaking changes in epiprocess soon
  • You want to co-develop features in epipredict and epiprocess

Change explanations for reviewer

Reviewing this should probably wait for #430 to merge

Draft ready for review:

• Landing Page
• Getting Started
• Customized Forecasters
• Reference
  • Using the add/update/remove and adjust functions
  • Smooth Quantile regression
• preprocessing and models examples
• backtesting forecasters

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves https://github.com/cmu-delphi/forecasting-team-meta/issues/26

[dajmcdon]

/preview-docs

[dsweber2]

/preview-docs

[github-actions]

🚀 Deployed on https://67980f5eb407dc9c1185cd57--epipredict.netlify.app

[dshemetov]

Our setup is generating docs in dev/, so the link is off, this works:
https://6792d4953137ef0ce0547a4f--epipredict.netlify.app/dev/

Also FYI: the bot edits its own comment for links. Each preview is a separate link and the links stick around for like 90 days. You can see the previous links in the comment edit history.

Edit: this has been fixed on main so is no longer necessary

[dsweber2]

/preview-docs

[dsweber2]

/preview-docs