[e.cash] Add start of brand page

Summary: adding first half of brand page. Will add the logo links in another diff

Test Plan:
preview the site and navigate to the ecash brand page
Check everything looks good on mobile and both themes

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D13928