docs: give io_result a page of its own and annotate the early examples

[sgerbino]

From the review result:

io_result needs to be front and center in the Capy docs -- it is in the Corosio docs, but it's buried in the task section of the Capy docs. That is, of course, partially how it is used as an unnamed return value split into elements. Comments in some early examples with a link to the reference could help.

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

io_result is covered inside 4a.tasks.adoc with no nav entry of its own. The landing-page Quick Example opens with auto [ec, n] = co_await stream.read_some(...) with no comment or link explaining the pattern; the quick start and first example page do not introduce it either.

• Dedicated page or top-level nav entry for io_result / io_task
• One-line comment plus reference link in the landing-page and quick-start examples