Add docs/threading-model.md and wire up references

[Copilot]

The threading model documentation was missing a canonical Markdown home. threading.html was not present in the repo, so only the creation and linking work was needed.

Changes

• docs/threading-model.md (new) — Covers both threading strategies (SocketAcceptor/SocketInitiator single-threaded vs. ThreadedSocketAcceptor/ThreadedSocketInitiator thread-per-session), queue capacity / watermark configuration, a comparison table, example usage, and thread-safety guidance.
• quickfixj-core/src/main/doc/usermanual/index.html — Added "Threading Model" entry in the Using QuickFIX/J list via relative path (../../../../../docs/threading-model.md).
• README.md — Added a ## threading model section linking to the new file, consistent with the adjacent ## configuration options section.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Delete unused threading.html documentation (use only Markdown file)</issue_title>
<issue_description>The new documentation for the QuickFIX/J threading model should exist only as docs/threading-model.md in GitHub markdown format.

Please remove the duplicate HTML version (quickfixj-core/src/main/doc/usermanual/usage/threading.html). Only the markdown should remain and be maintained going forward, as it is sufficient for both contributors and users.

Checklist:

• Ensure docs/threading-model.md exists and is up to date
• Remove quickfixj-core/src/main/doc/usermanual/usage/threading.html if present
• Update any README or user manual index references to point only to the markdown documentation, if applicable
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Delete unused threading.html documentation (use only Markdown file) #1172

---

📍 Connect Copilot coding agent with Jira, Azure Boards or Linear to delegate work to Copilot in one click without leaving your project management tool.