Add sub-headings to the "Potential API" explainer section

[domenic]

The "Potential API" section is great, guiding the reader through various increasingly-complex cases. I think it would benefit from small thematic breaks so that folks could tell when you're switching gears.

In particular I'd imagine:

• Acquiring and opening a port
• Writing and reading from a port
• Reading in a loop (or this could be part of the previous section)
• Handling non-fatal errors
• Using transform streams
• Sending signals (or, perhaps, "Other parts of the port API", and you could expand it to cover getInfo/setSignals/getSignals?)
• Watching for changes to the port

Other potential enhancements:

• Remove the "Potential" from the API. It seems reasonably settled by this point.
• Add a table of contents to the explainer. I use https://www.npmjs.com/package/doctoc (not as part of the build process, just locally and manually); if you do the comments and headings in the format seen in https://raw.githubusercontent.com/WICG/portals/master/README.md then just running doctoc README.md will update it.