[website] Add dedicated upgrade page

Summary:
This will allow us to use a dedicated link (bitcoinabc.org/upgrade) for upgrade specific communications.

Also includes some more answers to questions users may have.

Test Plan: Built and ran it locally, tested links.

Reviewers: majcosta, Fabien, #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9485