Document the Debian package in the install docs

[IgorKasianenko]

What

Debian accepted mistral-common into the archive as python3-mistral-common, so it can be installed with apt rather than into a virtual environment. #287 asks for that to be documented.

Change

Adds a Debian section to docs/usage/install.md, between Pip and From source. Docs only, no code.

Claims in the section, and where they come from

• Package name and install command — packages.debian.org/sid/python3-mistral-common; binary package python3-mistral-common, architecture all, source package python-mistral-common.
• "maintained by the Debian Deep Learning Team, not by Mistral AI" — the tracker page lists that team as maintainer, with Mathieu Baudier, who opened docs: python3-mistral-common accepted in the Debian archive #287, as uploader. The note is there so that packaging problems are reported to Debian rather than to this repository.
• "ships the base library only" — the package's Depends are python3-jsonschema, python3-numpy, python3-pil, python3-pydantic, python3-pydantic-extra-types, python3-requests, python3-tiktoken and python3-typing-extensions. That is exactly the dependencies list in pyproject.toml. None of opencv-python-headless, soundfile, soxr, sentencepiece, llguidance, huggingface-hub or fastapi is pulled in, so none of the extras are covered.

No version number and no suite name is written into the docs, because both move. The section links to the package page instead.

One thing to be aware of

Version 1.11.7-1 is marked for autoremoval from testing on 5 September 2026, caused by release-critical bugs in transitive dependencies (fastapi, httpcore, jupyter-notebook, node-playwright, node-yarnpkg, towncrier). That affects testing only, not unstable, and the section names no suite, so the text stays accurate either way.

Testing

Markdown only. The pre-commit hooks here (actionlint, ruff-check, ruff-format, uv-lock) do not cover markdown. mkdocs.yml does not enable the admonition extension, so the section uses plain headings and a fenced block, matching the rest of the file.

Closes #287.

Should the README's "How to use it ?" section carry the same line, or is the docs page enough?