[WEBSITE] Add download links for Electrum ABC releases

Summary:
The code is adapted from the Bitcoin ABC releases page, with some simplifications (no release notes included, no #version anchors).
For the Windows relase, I only link to the ...-setup.exe file, as the other 2 options confuse most users.

The 4.3.x versions, which are not using eCash and XEC, are excluded from the list.

Test Plan:

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Maniphest Tasks: T2197

Differential Revision: https://reviews.bitcoinabc.org/D11113