docs and an example: a lexicons page and a prediction in a shipped example's Ops

[lex00]

Part of #2355. Depends on the plugin contract and the first engine adapter sub-issues. Docs and an example.

What

The lexicon needs a docs page in the shape of the existing lexicon docs, and the prediction needs to show up in a shipped example's Ops, not only in tests.

Do

Add a page under the lexicons docs describing the plugin contract, the coverage table, and the refusal behaviour. Add the predictBehaviour Op to one shipped example's Ops, alongside its existing pipeline.

Proof

The docs site builds with the new page. The example's Ops run end to end with the prediction included.

Refs #2355.

[lex00]

This issue is the docs-and-an-example sub-issue for a lexicon that no longer exists. #2390 deletes lexicons/augur/ and moves the capability into core: the rows now belong to the lexicon that owns each entity type, and predictBehaviour() is core's rather than a plugin member.

So the work here splits three ways rather than being done as written:

The lexicons page and the behaviour-contract page are already updated in #2390, because deleting a lexicon that a docs table names is not optional.

The per-lexicon coverage page — which entity types reach an engine and which are declared unmapped — has no single home any more. It is now three sections owned by aws, k8s and terraform, generated from each lexicon's own behaviourKinds. That is real work and worth keeping, but it is a different issue from the one filed.

The example is the part that survives unchanged in spirit: a project that declares an estate, states a traffic level on a BehaviourOp, and shows a prediction and a declared-versus-live delta. It needs no lexicon of its own now, which makes it a better example than the one this issue imagined.

Suggest closing this and filing the coverage-page generation and the example separately, rather than rewriting it in place — the epic it belongs to (#2355) reads as finished otherwise, and it is not: nothing emits the behaviour block on graph nodes yet, which is the gap that keeps a consumer on fixtures.