Use --shallow in doc build to prevent documentation.js from processing GLSL imports

[nbogie]

Resolves #8499

Changes:

Add --shallow to documentation build in docs script in package.json

shallow mode turns off dependency resolution, only processing the specified files (or the main script specified in package.json)
from documentation build --help

This seems like a good general policy, going forward.

Testing

1. The change reduces error reports from documentation.js from 19 to 0 during npm run docs. (It's no longer trying to parse .frag and .vert files imported by some of our .js files.)

2. The produced /docs/data.json file is the same before and after the addition of --shallow, except that elements are in different order. Identical once sorted.

(Why the order changed: I guess documentation.js was previously processing files in dependency-resolved order, after the first file. With shallow, it just processes files in order that the input files are given from the command-line glob.)

3. The reference website pages also build fine, as you'd expect, though i didn't diff website content before and after.

Here's a command-line to diff two JSON files after sorting them in-memory. Needs jq json tool installed.

diff <(jq -S 'walk(if type == "array" then sort else . end)' docs/data.json) \
     <(jq -S 'walk(if type == "array" then sort else . end)' other.json)

[davepagurek]

Good catch!