[website] Update services listed on eCash page

Summary:
Add exchanges section and some exchanges
Add ViaWallet explorer
Add Cashtab browser extension

Test Plan:
Built and ran it locally, tested the links.
Checked that sites have good eCash branding.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9752