docs: move the coroutine and concurrency introductions out of the main reading path #398

From the review result:

Consider moving the Introduction to concurrency and C++20 coroutines outside of this documentation. While it's refreshing to see the tutorial on these things it distracts from the goal of these docs which is learning the library. Most of the topics discussed there (aka futures) aren't ever seen again in most of the library docs. Or at least move all that to the bottom so it's not 'the first section you read' by following the links.

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

Today sections 2 (Introduction To C++20 Coroutines) and 3 (Introduction to Concurrency), nine pages total, sit in nav.adoc immediately after Quick Start and before any Capy content, and index.adoc routes new readers into them. The Futures page remains even though the landing page states Capy has no futures.

• Move both sections to an appendix (or out of the Antora tree)
• Update index.adoc "Next Steps" to route into Capy content first
• Reconsider the Futures page