[GG-1820] Document the SEC lifecycle API

[dgra]

Addresses: GG-1820

Background

• A client can create a Single-Email Campaign over the API and send a test of it, but the endpoints that decide who receives it, and that take a scheduled campaign back to draft, have shipped without public documentation.
• Publishing these docs is the go-live gate for that work: the REST endpoints are already live on production.

Modification

• Adds a Segments page, read-only. Segments are built in the app; this documents finding one and the id the audience endpoint takes.
• Documents setting a Single-Email Campaign's audience, including the filter shape, that [[]] means everyone, that segments with in and not_in is the whole vocabulary today, and that replacing an existing audience needs confirm=true.
• Documents reading an audience back, including the case that will be most common for a while: an audience built in the app whose rules have no form here yet, which comes back described rather than as a filter.
• Documents unscheduling, including that the send time is not preserved and that a repeat call is safe.

Result

• The SEC lifecycle is documented end to end for the operations that are live: create, set an audience, read it back, unschedule.
• Scheduling is not included; it is still in review.