docs: document async client (AsyncDatamaxi) in README and docs

[martinkersner]

Closes #162

Summary

Documents the async client (AsyncDatamaxi, added in #149/#150). Restructured per review feedback so async is not a bolt-on duplicate lump.

mkdocs docs site — real Sync/Async content tabs

• Enabled pymdownx.tabbed (alternate_style: true) in mkdocs.yml.
• Converted the Python example on every endpoint page into a Sync/Async tab pair (17 pages): all CEX pages, funding-rate, liquidation, open-interest, margin-borrow, index-price, premium, forex, naver-trend, telegram, and the index.md quickstart. Each Async tab is the awaited twin of the Sync tab (full async with + asyncio.run scaffold per page).
• Slimmed docs/async.md from a per-endpoint duplicate into a concept page: install the datamaxi[async] extra, lifecycle (async with / await client.aclose()), same DATAMAXI_API_KEY env var, coroutine/await rule, async next_request pagination, standalone AsyncTelegram/AsyncNaver, and the ::: datamaxi.aio.AsyncDatamaxi mkdocstrings reference. Points readers to the per-endpoint tabs. Still in the nav.

README — no tabs (GitHub/PyPI render them as raw text)

• Replaced the earlier bulky async block with a slim co-equal subsection right after Quickstart: one paragraph, the datamaxi[async] install, one runnable asyncio.run example, the lifecycle + async next_request caveats, a one-line mention of AsyncTelegram/AsyncNaver, and a pointer to the docs' Sync/Async tabs. API Reference stays sync-only. ToC keeps the "Async Client" entry.

Correctness fix (both tabs)

While writing the Async twins I found existing docs examples passing kwargs that neither the sync nor async SDK accepts (would raise TypeError): cex.token.updates(sort=...), cex.symbol.cautions(base=...), cex.symbol.delistings(base=...), cex.symbol.volume(exchange=...). Removed those kwargs from both the Sync and Async tabs so the examples are valid and true twins, and corrected the now-inaccurate cex-symbol note.

Test plan

• mkdocs build --strict passes clean (only a pre-existing unrelated 'unrecognized relative link LICENSE' INFO in index.md); verified Sync/Async content tabs render in the built HTML.
• All 17 Async-tab snippets validated with ast.parse.
• Every method/attribute/arg verified against datamaxi/aio/*: coroutine await, async next_request, env-var handling, and the standalone Telegram/Naver clients.
• Docs-only; no SDK code touched.