fix: Add async modules to the readthedocs API reference

[jsonbailey]

Overview

The readthedocs API reference (docs/api-main.rst) documented no async modules. This adds automodule entries so the experimental async surface shows up in the generated docs.

New entries:

• ldclient.async_client — AsyncLDClient
• ldclient.async_config — AsyncConfig
• ldclient.plugin — Plugin and AsyncPlugin (this module had no entry at all; a pre-existing gap for the sync Plugin too)

AsyncHook (ldclient.hook) and AsyncMigrator (ldclient.migrations) were already swept in by the existing :members: blocks, so they need no new entry.

These modules import without aiohttp, so the docs build needs no [async] extra. make docs builds cleanly and all three modules render.

Part of the async SDK documentation follow-up (SDK-2879). The async surface shipped experimentally in 9.17.0.

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Note

Overview
Extends docs/api-main.rst so the generated API reference includes the experimental async surface and a previously missing plugin section.

Adds Sphinx automodule blocks for ldclient.async_client (AsyncLDClient), ldclient.async_config, and ldclient.plugin (sync and async plugin types). No runtime or SDK behavior changes—docs build only, aligned with SDK-2879.

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