Add the uv installation method

[lntutor]

Fixes #436.

Adds a dedicated uv installation section using uv tool install did, while retaining the existing package, virtual environment, and plugin-extra instructions.

Validation:

• pre-commit run codespell --files docs/install.rst
• git diff --check

The full pre-commit environment could not finish locally because its existing mypy hook dependency set failed while building gssapi on macOS.

[lntutor]

Current head f698c02cd3e7aef52f6d4d9e6caf4af0fad98612 is mergeable, and the visible current-head checks are in a clean state: Read the Docs is green, the Packit RPM builds are green, the Testing Farm jobs are green, and the osh-diff-scan result is neutral rather than a failing gate. There are no review threads or review-body comments open on the PR. If this documentation addition still matches the intended direction, it looks ready for maintainer review/merge when convenient.

[psss]

Would it make sense to also shortly mention uv tool run did as the newly installed script is not automatically added to PATH? Might be useful for new uv users. @lntutor, what do you think?