Docs: code-only assets under `AssetFolders` are silently skipped unless reachable from `RootAssets` — build is green, `Content.Load` fails at runtime

[jpenberthy]

Release Type: Official Release
Version: toolkit 1.0.0-preview.62, Stride 4.3.0.2507
Platform(s): All (code-only projects)

Describe the bug (docs gap)
In a code-only project, an asset present under AssetFolders but not reachable from the
package's RootAssets set is silently skipped by the asset compiler: the build succeeds,
incremental builds report "up-to-date", and the first sign of trouble is a
NullReferenceException from Content.Load at runtime. Nothing in the code-only docs states
the reachability requirement. Of every trap we hit in a five-day code-only evaluation, this one
cost the most debugging time.

Expected behavior
The code-only guide documents the RootAssets reachability rule (and ideally the compiler
warns about orphaned assets).

Additional context
Happy to PR the docs paragraph.