Instructions for embedding Google-Sheet-based schedules in the schedule page

[emiliom]

We have a Google Sheet with multiple tabs that has the event schedules. These schedules are embedded in a website page. This year (OHW23), it'll be this page alone: https://oceanhackweek.org/ohw23/schedule.html

The individual tabs that will be embedded first needs to be "published to the web" before it can be embedded. In the Google Sheet, go to File > Share > Publish to Web. This pop up will appear:

Click Embed, then select the tab that you want to publish. Then click Publish and copy the text block that's shown. This block is a url enclosed in an HTML <iframe> tag:

That block is then posted as is into the Markdown page:

oceanhackweek.github.io/ohw23/schedule.md

Lines 7 to 13 in c5ce42d

	## Seattle in-person schedule
	<iframe width=700 height=600 src="https://docs.google.com/spreadsheets/d/e/2PACX-1vQDKxlU84Ih9aipNGxoNSDc-b1wuHV-DK2sTZT1YfuvFcaVadFqfLWqrU6AvZMu6IEwLmqKciBZrDmx/pubhtml?gid=539810478&single=true&amp;widget=true&amp;headers=false"></iframe>
	## Virtual schedule
	<iframe width=700 height=600 src="https://docs.google.com/spreadsheets/d/e/2PACX-1vQDKxlU84Ih9aipNGxoNSDc-b1wuHV-DK2sTZT1YfuvFcaVadFqfLWqrU6AvZMu6IEwLmqKciBZrDmx/pubhtml?gid=21983846&single=true&amp;widget=true&amp;headers=false"></iframe>

I've already done this for OHW23 (PR #272)! So, we probably won't need to do it again, unless we add an Australia schedule.