[FR]: Document how to work with CSS files published by @angular/material

### What is the current behavior?

Error encountered when trying to depend on a file within an npm dependency:

```
no such target '//:node_modules/@angular/material/prebuilt-themes/deeppurple-amber.css'
```

It's unclear how to depend on this file.

### Describe the feature

With `rules_nodejs` and `rules_sass`, I used to have this:

```starlark
sass_binary(
name = "styles",
src = "styles.scss",
deps = [
":styles_deps",
#"//:node_modules/@angular/material/prebuilt-themes/deeppurple-amber.css",
],
)

sass_library(
name = "styles_deps",
srcs = [
"//:node_modules/@angular/material/prebuilt-themes/deeppurple-amber.css",
# //:node_modules/@angular/material
],
)
```

It no longer works due to the build error above. Do the npm rules in `rules_js` make such files available as bazel targets? How/why not?

### Fund our work

- [ ] Sponsor our open source work by donating a [feature bounty](https://opencollective.com/aspect-build/)

Start with the shown styles.scss, sass_binary, and sass_library configuration and reproduce the missing-target error for Angular Material's prebuilt-themes/deeppurple-amber.css path. Review how rules_js exposes files from npm dependencies, then document whether this CSS file is a Bazel target and how it should be consumed; done means a newcomer can follow the documented setup without the reported error.

Written by the indexing model from the issue text.