fix: build tox documentation with system make - #490

The documented tox -e docs command resolves make as a Python package through uvx. That package provides no executable, so the docs flow exits before invoking the already available GNU Make or Sphinx.

Run make -C docs html directly and remove the unused uvx external allowlist entry. The existing Makefile still sets PYTHONPATH=../src and uses the original tox docs dependencies. No Makefile, dependency, package, or workflow changes are needed.

Validation:

• Exact original tox docs flow on the canonical base installs the declared dependencies, invokes uvx make, and exits 1 with Package make does not provide any executables.

• Exact repaired tox -e docs, with fresh generated output and no command override: exit 0 on Python 3.12.13, tox 4.64.8, Sphinx 7.0.1 and GNU Make 3.81. It processes all 44 sources and produces 47 HTML files. The 12,953-byte index.html exists and its title parses correctly.

• Config/code review approves; no runtime or other source bytes change. Earlier missing-tool attempts are kept separate from the executed original failure.

• Add the required pr490.bugfix.rst newsfragment using the actual PR number; Towncrier 25.8.0 check passes.

• The original Python workflow path filter excludes this tox/RST-only diff, so no Python run is reported for this head. The upstream CodeQL run requires maintainer approval. No skipped or absent test run is recorded as passed.