[website] Avalanche Post-Consensus launch blog post

Summary: Explain what's going on with the countdown and launch.

Test Plan:
Ran it locally, and read it through, tested links.
bundle exec jekyll serve --future

Reviewers: #bitcoin_abc, aimal

Reviewed By: #bitcoin_abc, aimal

Subscribers: Klakurka, Kronkmeister

Differential Revision: https://reviews.bitcoinabc.org/D11883