Wrap home page JSON-LD in @graph with a top-level @context (#19)

The home page injected its JSON-LD as a bare top-level array, so any
consumer treating the parsed structured data as a single object read
`undefined["@context"]` and crashed. Wrap the three entities in a
`@graph` with one top-level `@context` and drop the now-redundant
per-object `@context`.

Generated-By: PostHog Code
Task-Id: 4d331e29-d220-4346-b6c8-43e10e90a2be

Co-authored-by: posthog[bot] <206114724+posthog[bot]@users.noreply.github.com>

Author: posthog[bot]