[website] Improve the Electrum ABC introduction paragraphs

Summary:
Give a description of what Electrum ABC and mention some features, instead of defining it as an Electron Cash fork.
Remove mentions of splitting coins and copying Electron Cash wallets.

Test Plan:

./scripts/fetch_electrum_releases.sh
bundle exec jekyll serve
firefox http://127.0.0.1:4000/electrum

Reviewers: #bitcoin_abc, bytesofman, Fabien

Reviewed By: #bitcoin_abc, bytesofman, Fabien

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D11577