Fixes the skill installation instructions in `docs/src/ai/skills.md`.

The previous steps used a `git sparse-checkout` approach that was
inaccurate — the cloned repo would land nested inside the skills folder,
and the sparse-checkout path was wrong for repos that store skills in a
subdirectory. Replaced with a simple instruction to copy the skill's
folder into the appropriate location.

Release Notes:

- N/A