docs: document quitter and the other toolkit examples in the manual

[sgerbino]

From the review result:

Then I found, not in the docs, but in the examples from the repo the 'quitter' example [...] it's clear from the example that outside of task Capy has other not well documented 'toolkit elements'. [...] why leave this stuff in if it's not really documented?

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026.

quitter is a first-class library type (tested, reference-documented) but has no manual page and no nav entry; its design rationale lives in doc/quitter.md, a loose file outside the Antora tree that never renders. The same gap applies to async_mutex, async_event, async_waker (reference-only) and to examples that ship without a docs page (when-any-cancellation, fabrics, cuda, asio interop).

• Manual page for quitter (when to prefer it over task, shutdown pattern) plus a nav entry for the example
• Manual coverage for the synchronization primitives (async_mutex, async_event, async_waker)
• Move or merge doc/quitter.md into the Antora tree
• Docs pages (or nav entries) for the remaining undocumented examples