Show GH CLI command for starting build

[hugovk]

For #108.

Prints the command needed to start a build using the GitHub CLI: https://github.com/cli/cli

For example, it will output something like this, the second bit is new:

Go to https://github.com/python/release-tools/actions/workflows/source-and-docs-release.yml
Select 'Run workflow' and enter the following values:
- Git remote to checkout: hugovk
- Git commit to target for the release: 240b882b2a17621c1a32eceb9227447be55ffb84
- CPython release number: 3.14.0a6

Or using the GitHub CLI run:
  gh workflow run source-and-docs-release.yml --repo python/release-tools -f git_remote=hugovk -f git_commit=240b882b2a17621c1a32eceb9227447be55ffb84 -f cpython_release=3.14.0a6

(We can later further automate by actually calling this: we'll need to make sure RMs have the tool installed -- if that's an acceptable requirement. Otherwise, via API calls, which will need token management etc. But first let's just print it.)