Address Guidehouse feedback on landing page

[brookslogan]

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

This addresses some feedback we received from Guidehouse on the landing page; here are my notes:

• We should provide more context on landing page: why, what need, what ?? addresses; highlighting more e.g. the top 2 novel features.
• Useful in landing page to have a little more detail e.g. about 7dav-ing?
• Add to landing page the expected level we expect people to have to use the package (we expect people to be able to use these packages in R that are very common e.g., dplyr and tidyr).

There is some slightly-awkward link spam for {epiprocess} and {epipredict} so that we retain braces around all package names; if we don't manually turn them into links, then they will automatically be turned into links anyway and have the braces dropped (and a slightly worse link for epipredict, probably because we list the repo URL before the github.io URL in the DESCRIPTION file).

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

[brookslogan]

Rebased & force-pushed to resolve DESCRIPTION + NEWS.md conflicts, plus improved a little wording in NEWS.md.

[nmdefries]

Looks good. Suggestions are optional.

[nmdefries]

suggestion: give broad intro before detailed list

Suggested change

	The [`{epiprocess}`](https://cmu-delphi.github.io/epiprocess/) package provides
	The [`{epiprocess}`](https://cmu-delphi.github.io/epiprocess/) package introduces a common data structure for epidemiological data sets
	measured over space and time, and offers associated utilities to perform basic
	signal processing tasks.
	The package provides the following tools:

[brookslogan]

That intro intro's not accurate, I've taken a shot at an accurate one.