Update CONTRIBUTING.md to use Github `Generate release notes` button

[maratal]

This button does pretty much the same thing as the github_changelog_generator script described now in the CONTRIBUTING guide. No need for this script anymore since this functionality is now built-in.

┆Issue is synchronized with this Jira Task by Unito

[lawrence-forooghian]

I remember that we discussed this internally but I don't remember if it was in Slack (can't find it) or just in a standup. Do you remember? I remember mentioning that it would not fit well into our current release process because currently we write the changelog before creating a tag.

[maratal]

Recent similar discussion - https://ably-real-time.slack.com/archives/C03JDBVM5MY/p1713542539425589 Owen somehow managed to do that in ably-js.

[lawrence-forooghian]

Ah cool, seems like you can type in a dummy tag name in the "choose a tag" field 👍