[DOCUMENTATION] guide for uploading WM ext public key to wallets

[tselit]

Is your feature request related to a problem? Please describe.
As a user of Web Monetization, I need guidance on how to upload a public key to my Open Payments-enabled digital wallet, which associates a Web Monetization browser/extension to my digital wallet address.

Describe the solution you'd like
Update the Guides section of the WM docs with a step-by-step guide for uploading the WM extension's public key to each of the compatible digital wallets:

• Fynbos
• Gatehub

Describe alternatives you've considered
We considered providing custom documentation for the Web Monetization extension.
However, adding to the on https://webmonetization.org/docs/ would be more consistent with the existing WM guide documentation.