[website] add a couple of keywords to the electrum download page for SEO

Summary:
Added keywords:

• XEC
• open source
• official

Test Plan:
Check the result locally
make run
http://127.0.0.1:8080/electrum/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13251