[electrum] documentation overhaul and dedicated CONTRIBUTING.md

Summary:
Consolidate all developer documentation in a dedicated CONTRIBUTING.md file.

Rewrite the README.md to better describe the project and provide only info targeted at users (most technical/dev info moved to CONTRIBUTING.md).

Make the release notes file only about release notes (credits moved to README.md).

This diff mostly moves paragraphs and section between files, with only minor additions:

• general description of the project and features in README.md
• link the release notes in README.md
• mention the functional tests in the "Running tests" section. This was previously undocumented.

Test Plan: proofreading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14259