docs: add CHANGELOG and seed changelog/ folder

Document the 1.0.0 release in CHANGELOG.md (Keep a Changelog format,
SemVer): natives, body builders, response headers, cancellation,
per-request timeout, auth helpers, cookies, mTLS, redirect policy,
security defaults, build tooling, CI workflows, and license.

Seed changelog/v0.x.md as a placeholder noting that no public 0.x
releases exist, so the folder mirrors the layout used by the
sibling plugins and is ready to receive legacy entries on future
major bumps.