Document the async client (AsyncDatamaxi) in README and docs

[martinkersner]

Problem

Async support (AsyncDatamaxi in datamaxi/aio/, added in #149 / #150) is completely undocumented. The README has zero async mentions, and the mkdocs site under docs/ documents only the sync Datamaxi client.

Users have no way to discover the async client or learn its usage (async context manager, await, aclose()).

Scope

• README.md — add an async usage section mirroring the sync Quickstart: from datamaxi.aio import AsyncDatamaxi, async with, await client.cex.candle(...), etc. Add it to the Table of Contents.
• docs/ (mkdocs site) — document the async client so it renders on the docs site. Match the structure/style of the existing sync pages.

Notes

• Public surface: AsyncDatamaxi (async context manager: __aenter__/__aexit__, plus aclose()), exported from datamaxi/aio/__init__.py. The resource tree mirrors the sync client (feat: expand async client to the full resource tree (#142) #150).
• Keep examples runnable and consistent with the sync examples (same endpoints/args).

Done when

README and the mkdocs docs both show how to install/import and use the async client, with at least one runnable async example, and async is discoverable from the README ToC + docs nav.

Related: #154