docs: make "Why not std::execution" a peer design page and link the comparison paper #403

From the review result:

The design section has 'Why not Cobalt?', 'Why not TooManyCooks', etc -- given where we are there should be 'Why not std::execution'. And while we're there there's an example that bridges the models users might be coming with some knowledge of std::execution. Pointers to papers that describe this are a valid approach.

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

The content exists but is buried: "Why Not std::execution (P2300)" is a == heading inside 9k.Executor.adoc rather than a nav entry beside WhyNotCobalt/WhyNotTMC. The sender-bridge example page (8o) exists and is in the nav. papers/B1001.comparison-to-p3552-task.md is linked from nowhere in the docs.

• Promote the section to its own 9.design page with a nav entry
• Cross-link the sender-bridge example and the P3552 comparison paper
• Deduplicate the "context flows forward" argument that currently appears in three pages