Document release flow in README.md

[manzt]

Documents the full release flow via the GitHub UI.

Releases are managed via the GitHub UI:

1. Draft a new release
2. Create a tag in the format v[major].[minor].[patch].
   • Note: You can either create the tag in the GitHub UI or select an existing tag created via the Git CLI. The tag triggers publishing to PyPI.
3. Generate release notes to summarize changes from merged PRs, and edit.
   • Documentation and CI changes don't belong in release notes
4. Add details for significant changes or migration steps, and ensure titles are clear and consistent.

Also removes CHANGELOG.md to avoid redundancy. Points to GitHub Releases instead and updates PR template with
instructions.

[pkerpedjiev]

As per the Slack conversation, I recommend making 2. a sub item of 1 so that it's clear that the tag is created during the release process.

[manzt]

Since all the steps that follow are a part of "drafting a release", I reorganized things. Let me know hwat you think.

[pkerpedjiev]

Looks good 👍