[Docs] Updated app URL at docs.cashtabapp.com

Summary: Replaced cashtabapp.com with cashtab.com throughout docs.cashtabapp.com Cashtab Web Wallet overview. Please advise on whether to change the URL in docusaurus config.

Test Plan: Running grep -r 'cashtabapp' /web/docs should return nothing.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D10644