docs: ship the public changelog entries in the doc package

[BryanFRD]

Closes #1303

FerrLabs/Changelog is being retired: each product keeps its public changelog entries next to its code, and ferrlabs.com now shows only FerrLabs entries. ferrflow.com still downloads FerrLabs/Changelog from main at build time.

• docs/site/changelog/ holds the 82 FerrFlow entries, copied byte for byte from FerrLabs/Changelog. docs/site is the @ferrflow/doc package, so they ship with the next release; FerrFlow-Cloud will read them from the package instead of fetching the Changelog repo. The site docs validator only looks at docs-* directories, so the new one does not affect it.
• CONTRIBUTING now says where an entry goes and when one is due: in docs/site/changelog/, in the same pull request as the change.
• The README's Changelog link pointed at ferrlabs.com, which no longer lists FerrFlow entries; it now points at ferrflow.com/changelog.

docs: as CONTRIBUTING asks for a documentation-only change; the entries reach the package with the next feat or fix release.

[ferrfleet]

Nothing blocking. docs/site/package.json has no files allowlist and there is no .npmignore, so changelog/ ships in @ferrflow/doc as intended. The CONTRIBUTING frontmatter spec matches the keys the entries use, and the README link fix is correct.

I did not review the entry prose, since it is a byte-for-byte copy of FerrLabs/Changelog.