Document release flow in contributing guide

[stinodego]

We will slightly change the release flow:

• Create PR with update to version numbers
• Merge the PR
• Manually trigger the release using the GitHub UI
• Have the CI job trigger on the tag created by the release

This can work for both Python and Rust. I recommend we add a GitHub Actions workflow to do the Rust release as well.

To do:

• Update contributing guide
• Add Rust release workflow

[stinodego]

We should still add a Rust release workflow. I'll make a separate topic for that.

[stinodego]

Closing in favor of #14366

Once we have that set up, it can be documented.