docs: add payment pointer to glossary

[melissahenderson]

Add "payment pointer" to the glossary and describe the relationship between it and a wallet address. We might want to link to paymentpointers.org... but that site needs to be updated, so maybe we hold off. We can discuss.

[melissahenderson]

This is the definition that's going to be in the Web Monetization doc's glossary. It's not appropriate for Open Payments word-for-word, but might help when crafting.

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A payment pointer is a secure, unique identifier that’s assigned to payment accounts that use Interledger’s Simple Payment Setup Protocol (SPSP) to exchange payment information. The shorthand for a payment pointer starts with $. For example, $wallet.example.com/alice.

As a supporter of web monetized content, you’ll enter your payment pointer into your Web Monetization extension to link the extension to your wallet.

To web monetize your own content, you must convert your payment pointer to its URL equivalent because the monetization element does not support the $ in the href value. Enter your payment pointer into the field provided on paymentpointers.org to get your URL.