Update sound-usemic and sound-usespeakers

The information in the pages on selecting mics and speakers is a bit outdated, and doesn't really cover the kinds of issues people come across. Things to note, in no particular order:

• Built-in speakers are a thing that exist. These pages talk about speakers like they're always some external thing you plug in like you're living in the 90s.
• Bluetooth speakers, mics, and headsets are things that exist. We should link off to a page about pairing.
• You have to select your speaker and mic separately, even if they're the same device (like headsets). These two pages should link to each other a bunch.
• Thunderbolt docking stations can add audio devices. If you plug headphones into the docking station, you probably won't get audio if you've selected the laptop's audio device.
• Although USB and Bluetooth headphones/mics appear as separate devices, things plugged into stereo jacks often do not. That's often an automatic hardware switcheroo.
• Oh yeah, we should probably just call them stereo jacks, or maybe audio jacks. I'm a recovering audiophile, and I can go on and on about the differences and the correct terminology. Nobody cares.
• IMPORTANT: Lots of apps, including web apps, have their own settings. So even if you selected your speakers or your mic in GNOME, you may very well have to select them again in Google Meet, Bluejeans, BigBlueButton, and Zoom.