[Cashtab] Update cashtab docs links to point to web wallet docs

Summary: Cashtab documentation links from Onboarding and Settings pages point to docs landing page. Update to point to Cashtab web wallet docs page. Update unit test snapshots to match.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9221