docs: Add note about `path` in `extensions.toml` (#25778)

This PR adds a note about the `path` field in `extensions.toml` and how
to use it.

Suggested in https://github.com/zed-industries/extensions/pull/2128.

Release Notes:

- N/A
c05ce882e9Marshall Bowers committed on 2/27/2025, 9:47:01 PM· committed by GitHubparenteaf3949
1 file changedLine totals unavailable