[website] Airdrop Calculator article

Summary: As per @Mengerian request in T2185, an ABC blog post based on Cassandra video: https://www.youtube.com/watch?v=iuIljw9g8h0

Test Plan:

1. bundle exec jekyll serve
2. read through article

Reviewers: Mengerian, bytesofman, #bitcoin_abc

Reviewed By: Mengerian, #bitcoin_abc

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D11353