Fix #434: Tooling: local playground UI for exploring an index

[Mr-Neutr0n]

Fixes #434

Added a section about the local playground UI to the getting started documentation as requested in the issue.

No local test suite detected.

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[Mr-Neutr0n]

Closing this, and flagging it clearly rather than quietly: this PR documents a command that does not exist.

It adds a "Local playground UI" section describing moss playground, a default port of 3210, and a list of things the UI lets you do. None of that is real. There is no playground command in this repo — the only matches for the word are unrelated READMEs in the voice-agent examples and one Swift file. #434 is a feature request ("Proposed solution: Ship a local playground"), not a description of something shipped.

So the PR was wrong twice over. Merging it would have (a) told users to run a command that would fail, with a specific port number that came from nowhere, and (b) auto-closed #434 via Fix #434, marking an unbuilt feature as delivered.

That's my error and a bad one — writing docs from an issue's proposed solution as though it described existing behaviour. Sorry for the review time.

#434 should stay open. If a playground does get built later, the docs are the easy part and should be written against what actually ships.