Add Prunerr to Useful Tools

[helliott20]

Adds Prunerr to the Useful Tools page, alongside Deleterr in the same media-cleanup category.

Prunerr is a media library cleanup tool for Plex/Sonarr/Radarr that reclaims disk space by removing unwanted content based on customisable rules, with a review-and-approve deletion queue, grace periods, a disk-pressure reactive mode, and outbound webhooks (incl. Home Assistant). MIT licensed, actively maintained, Docker multi-arch.

Added to both the table of contents and the body.

[helliott20]

Note on the failing pre-commit.ci check: it isn't caused by this PR. This PR changes only useful-tools.md, but markdownlint is failing on pre-existing MD040 violations (fenced code blocks missing a language) in unrelated files:

• lidarr/tips-and-tricks.md:25, :40
• lidarr/importing-existing-library.md:41, :47

pre-commit.ci lints the whole repo, so it trips on those regardless of this change. I've opened #450 to fix them — merging that should turn the check green here. The Prunerr addition itself is lint-clean (MD013/line-length is disabled in the config, and no code blocks were added).