Add video and Settings docs for Packet Capture features

[philrz]

I wanted to add some docs for the pcap-related Settings I added recently, but that made me confront the fact that we haven't produced updated docs about Zui's overall pcap functionality since the "Brim app" days. Therefore I went ahead and created a Feature Tour video and have linked to it from a new docs page created in this PR that also covers those new Settings.

If you want to see the rendered docs, I've put them on a personal staging site at https://jocular-choux-3cf6d8.netlify.app/docs/features/Packet-Captures. The "DOCS" hyperlinks in the updated Settings menu in this branch will be broken until this PR merges and the production https://zui.brimdata.io docs site is updated with these changes. However, you can confirm the hyperlinks are ready to work by clicking on them using a Dev build from the branch and then swapping in hostname jocular-choux-3cf6d8.netlify.app in place of zui.brimdata.io.

If this gets approved, I'll plan to hold it open until right before I'm prepping the next GA release. Some final steps I'll take upon merging:

• Do a final check that the "DOCS" hyperlinks in the app work correctly
• Make the YouTube video "Public" (right now it's "Unlisted" while this PR is being reviewed)
• Mention the video/docs on the Zeek community Slack, since this provides a good opportunity for that user base to become reacquainted with Zui

Closes #2705

[jameskerr]

Looks great Phil!