Investigate ways to enable previewing of unreleased changes

[frankieroberto]

Currently it’s a little awkward to preview unreleased changes, for example in a branch of the Service manual website (where we might be drafting guidance for upcoming changes), or to test unreleased changes in services or prototypes.

Options to explore could be:

• publishing pre-release (or 'release candidate') packages using the -rc.1 type suffixes
• enabling the package to be installed using a GitHub reference to a branch within package.json, eg `"nhsuk-frontend": "nhsuk/nhuk-frontend#branchname"
• using npm link - this can be done locally, but is there a way to do it temporarily on a deployed environment too?
• anything else?

The GOV.UK Design System team seem to do a combination of the first 2 options:

• Publishing a preview
• Publishing a pre-release