docs: add information about the release process

[adamconnelly]

Description of the change

I've added documentation to the repo about how the release process works, how new Pulumi versions are handled, as well as where the images are pushed to.

Type of change

• Bug fix (non-breaking change that fixes an issue);
• New feature (non-breaking change that adds functionality);
• Breaking change (fix or feature that would cause existing functionality to not work as expected);
• Documentation (change not affecting any of the images);

Checklists

Development

• The affected image builds in your local environment;

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached;
• This pull request is no longer marked as "draft";
• Reviewers have been assigned;
• Changes have been reviewed by at least one other engineer;