[website] link to the bitcoinabc.org html version of electrum release notes

Summary:
It is better to not send people to an external website for a document that now exists on our own website.

Also display only 5 past releases. I occasionally ask users to try a previous release for troubleshooting a bug, but 5 releases should be plenty enough.

Test Plan:

scripts\fetch_electrum_releases.sh
make run

Preview @ http://127.0.0.1:8080/electrum/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14272