Automate publishing to npm

> How does our npm package reflect changes to our repo?

Right now, it's manual publish- you have to `cd` into `packages/gatsby-theme-iterative` and run a `yarn publish`- ideally also changing the dependency version in the `example` sibling package but with patch upgrades semver will still use the latest version.

We definitely need to automate npm publishes, maybe not with every master merge but at least with a manual workflow run button that pushes the version bump commit.

_Originally posted by @rogermparent in https://github.com/iterative/gatsby-theme-iterative/issues/22#issuecomment-1142404932_

Start in packages/gatsby-theme-iterative and compare its current yarn publish process with the example sibling package's dependency version. Define a manual workflow entry point that publishes the npm package and pushes the version bump commit; done means the package can be released without the current manual command sequence.

Written by the indexing model from the issue text.