docs: add promotion instructions to "working with stages" doc

[krancour]

The "Working with Stages" doc has detailed instructions for doing many other things Stage-related, but under "Promoting Freight to a Stage," only has a placeholder.

We should add UI-based and CLI-based instructions for this.

Consistent with the rest of the page, the UI and CLI-based instructions can be on different tabs, with the UI-based instructions selected by default.

Note that this should show the "Stage first" method of promotion -- i.e. clicking the promote button on a Stage and then picking Freight from the timeline.

I am opening a separate issue for adding "Freight first" promotion instructions to the "Working with Freight" doc. The two sections can link to one another.