[e.cash] Add upgrade page

Summary: Make a page where people can find information about upcoming upgrades

Test Plan:
Ran it locally to check rendering looks good
Read through the content to make sure it's correct

Reviewers: johnkuney, Fabien, #bitcoin_abc, bytesofman

Reviewed By: Fabien, #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14616