[website] Add Avalanche development update blog post
Summary: Give some more info to people who are curious about what's going on.
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