Fix FFmpeg core documentation links

[tobiaspampinella]

Summary

Small documentation cleanup related to libretro core documentation coverage.

• Link the FFmpeg entry in the core list to its existing library page.
• Fix stale self-links in docs/library/ffmpeg.md so they point to the local sections.
• Correct dependant -> dependent in the FFmpeg geometry/timing section.

Verification

git diff --check
rg -n "\.\.\/ffmpeg|dependant|formats\(" docs/library/ffmpeg.md docs/guides/core-list.md

Related bounty issue: libretro/libretro-meta#79