docs: link the benchmark suites from the documentation #402

From the review result:

Where is the linkage to Capy performance benchmarks? [...] since Capy is in part a framework, maybe there could be a site for contributed benchmarks for libraries that build on Capy.

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

bench/ contains coroutine-overhead, allocation, capy-vs-stdexec, and capy-vs-beman suites, but nothing in doc/modules references them; the only benchmark mentions in the docs are prose without links.

• Add a benchmarks page (methodology, how to build/run, current results)
• Link it from the nav and from the design pages that cite benchmark numbers
• Decide where contributed downstream benchmarks would live