[website] Add Avalanche development update blog post

Summary: Give some more info to people who are curious about what's going on.

Test Plan:
Read it, make sure info is accurate.
Ran locally to make sure it looks good and links work.

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: aimal, Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D11402