Add covid_naat_pct_positive_7dav to the dsew documentation

[krivard]

We've apparently been merrily serving test positivity data from the DSEW Community Profile Report without telling anyone what it is or what it means. We should add an entry to the signal table for covid_naat_pct_positive_7dav.

• Source file
• Rendered file
• Indicator source

[nmdefries]

Added to signal spreadsheet. Added to documentation site in #1404.