docs: convert @libp2p/gossipsub README to generated format

[tabcat]

packages/gossipsub/README.md is still the original ChainSafe hand-written README — it was never converted to the monorepo's aegir docs generated layout (compare packages/floodsub/README.md). Because packages/gossipsub/src/index.ts has no @packageDocumentation block, nothing regenerates it, and it has drifted: outdated ES-2017/node-10.x badges, a TOC "Lead Maintainer" link to a section that no longer exists, an API link to the archived js-libp2p-interfaces repo, and a MIT © ChainSafe Systems footer instead of the standard dual Apache-2.0/MIT.

• Add a @packageDocumentation block to packages/gossipsub/src/index.ts and regenerate via npm run docs to match packages/floodsub.