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

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:
https://github.com/oceanhackweek/oceanhackweek.github.io/blob/c5ce42d9f8de03fbf1175bc2c141182920af0c52/ohw23/schedule.md?plain=1#L7-L13

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.

Read ohw23/schedule.md lines 7-13 and the Google Sheets publishing steps described in the issue, then check PR #272 for the existing OHW23 embeds. Done means any newly required schedule tab, such as an Australia schedule, is published and its iframe is added to the relevant Markdown page.