Restructure API docs for consistency

[gijzelaerr]

Summary

• Merge Client docs (s7commplus.rst + async_client.rst) into a single client.rst so Client, Server, and Partner all follow the same pattern: recommended (s7) first, legacy (snap7) below
• Add s7.Partner to partner.rst (was only documenting snap7.Partner)
• Group low-level internal modules (connection, s7protocol, datatypes, discovery) under a separate Internals section in the sidebar
• Keep user-facing modules (Client, Server, Partner, Logo, Util, Types) in the main API Reference section

Before

API Reference
  S7 Client (recommended)   ← s7.Client + s7.AsyncClient
  Client (legacy)            ← snap7.Client (separate page)
  AsyncClient (legacy)       ← snap7.AsyncClient (separate page)
  Server                     ← s7.Server + snap7.Server (one page)
  Partner                    ← snap7.Partner only
  Logo
  Types
  Util
  Connection                 ← internals mixed in
  S7 Protocol
  Data Types
  Discovery

After

API Reference
  Client                     ← s7 + snap7, sync + async (one page)
  Server                     ← s7 + snap7 (one page)
  Partner                    ← s7 + snap7 (one page)
  Logo
  Util
  Types

Internals
  Connection
  S7 Protocol
  Data Types
  Discovery

Test plan

• Sphinx builds clean with -W (warnings-as-errors)
• Verify sidebar structure on ReadTheDocs preview
• Check cross-references still resolve (e.g. connecting.rst -> client.rst)

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