docs(api-playground): three REST endpoints since Deploy - the README and CHANGELOG counts

[acn-ericlaw]

What

• README: “the three REST endpoints” (Eric’s edit); CHANGELOG Unreleased item 4: “the /api/specs endpoints”, no count.

Why

• The Deploy endpoint of feat(api-playground): Deploy saves the loaded document into the specs folder (PUT /api/specs/{filename}, confirm before overwrite); the folder's demo.yaml replaces the example; labels Specs, MiniGraph, Browse; api.playground.specs #548 made a third endpoint, and two sentences written before it still counted two.

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