Update web app help tab and add demo video

[Rnaught] / inst / web / templates / content / help / serial-interval.html

<p>
  The <a href="https://en.wikipedia.org/wiki/Serial_interval" target="_blank">serial interval (SI)</a> is a parameter
  required by most of the estimators, and can also be estimated by some of them (if not specified). For the latter, "—"
  is reported in the "Serial interval" column of the estimates table, and the estimated value is shown next to the
  <em>R</em><sub>0</sub> result(s) as "(SI = ...)".
</p>
<p>
  The SI is defined as the average time between successive infections in a chain of transmission (i.e., the time between
  the infection of an infected individual and their subsequent transmissions).
</p>