Link to docs for uploading WM ext public key to Fynbos or Gatehub wallets

[tselit]

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

The goals is to have a "learn more" link on the UI of the extension to the docs - refer to #573

[tselit]

To do: ask Tech. Writing team
To do: move to WICG project

[tselit]

Created ticket WICG/webmonetization#459 and assigned to the Tech Writing team.

[tselit]

Dependency on ticket for the Docs: WICG/webmonetization#459

[tselit]

Melissa aims to start after 2 Aug (if we're confident that no further UI changes are planned), and is comfortable that the doc update will be ready on/before the 13 Aug beta release.

[tselit]

Prepared the following docs and resources to improve support for beta testing:

• Get started with the WM extension: https://docs.google.com/presentation/d/1kYgvm59w5KE8sdDuFM90m6hmCg_itCS-/edit#slide=id.p3
• Test the ILF's beta extension: https://docs.google.com/document/d/1nn-xVpGrrQv0aOBTyQr22I0K_Wd1z0cfLVKMsAOVKpw/edit
• GateHub wallet guide for Web Monetization: https://docs.google.com/document/d/1jBJuw86Scc_8mL8Szd-6ZCcwuOfI58UJvVhDRIjZsVI/edit