[electrum] update links in the documentation

Summary: Fix a dead link in the old release notes, update some links in the electrum README

Test Plan:
I checked that at least on github the RST link to md docs work properly. The new link to the main contributing.md can be tested only after this lands.

I also checked that links on the bitcoinabc.org work (https://www.bitcoinabc.org/electrum/contrib/release.html)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14241