why: Cut the 0.1.0a39 release: sphinx-autodoc-fastmcp targets FastMCP 4
and documents a live server's tools, prompts, resources and resource
templates, and the extension's warnings now obey Sphinx's own strictness
controls.

what:
- Date the 0.1.0a39 CHANGES section (2026-09-06) and add its lead
  paragraph
- Open a fresh 0.1.0a40 unreleased placeholder
- Bump version 0.1.0a38 -> 0.1.0a39 across the workspace root, every
  packages/*/pyproject.toml and its __init__, and the two tests that
  assert the workspace version
- Refresh uv.lock

The demo server's docstring keeps 0.1.0a38 as an illustrative argument
value; it names an example release to fetch rather than tracking this
one.