docs: clarify id, filePath, and pathname in STATIC_FILE adapter output (#92227)
## What
Adds documentation for the `id`, `filePath`, and `pathname` properties
in the `STATIC_FILE` adapter output type.
## Why
These three string properties were previously undocumented or had
ambiguous descriptions. Based on feedback from the team:
- `id` is a unique identifier for the static file output
- `filePath` is the absolute filesystem path to the built file
- `pathname` is the routable URL pathname
## Changes
- Added JSDoc comments to the `STATIC_FILE` type definition in
`build-complete.ts`
- Updated the MDX documentation in `output-types.mdx` with clearer
inline comments