Add per-package API reference docs

[nficano]

TS SDK has docs/packages/*.md. Add docs/packages/arcp-{core,client,runtime,jetty,spring-boot,jakarta,vertx,otel,tck}.md documenting each Maven artifact's public surface and idiomatic usage.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)

[vinay-852]

Hi @nficano,
I’d like to work on this!. Can you assign this to me.

[nficano]

No need! just fork and create a PR 🙂 Thanks!