Add watch-only wallets to common use cases (BitcoinDesign#1136)

* Add watch-only wallets to common use cases

Several changes to the common use cases page, which was last updated 2 years ago:

- Addresses BitcoinDesign#1063 and adds a paragraph about watch-only wallets.
- Adds a few additional links to new content that has been added to the guide since this page was last touched.
- Standardizes link formatting to properly use relative URLs

Since AI is all the rage these days, I had GitHub Copilot write the added copy. It was about 75% good-to-go and just required some tweaks.

Closes BitcoinDesign#1063

* Minor copy tweaks based on feedback.

Author: GBKS