[website] Fix redirect for links with a trailing slash

Summary:
This should redirect from the following urls:

• https://www.bitcoinabc.org/2022-09-07-avalanche-staking-tutorial.html
• https://www.bitcoinabc.org/2022-09-07-avalanche-staking-tutorial
• https://www.bitcoinabc.org/2022-09-07-avalanche-staking-tutorial/

Test Plan:

bundle exec jekyll serve

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D12322