[website] Add info for people who forgot to upgrade their nodes

Summary: Version 0.25.6 includes the checkpoint that will cause the node to reconsider the fork block, and get them back onto the correct chain tip.

Test Plan: Ran site locally, looks good.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D11479