Add Alby wallet connection instructions

[bumi]

Alby is a browser extension that makes it easy to use lightning from the browser.
Alby can connect to LND directly or to a LNDHub.
We should offer Umbrel users instructions on how to connect Umbrel to the browser extension.

As also discussed in: getumbrel/umbrel-lightning#27

[jonsyu]

Closing because we've recorded this request - thanks for the suggestion (trying to find a better place to store these type of documentation requests as well)