docs(developer): add description of npm-run to run locally installed npm scripts

[BendingBender]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe: improve docs

What is the current behavior? (You can also link to an open issue here)
A description for a convenient common option to run locally installed npm scripts via npm-run is missing. Instead, a manual workaround which involves more complicated setup steps is described.

What is the new behavior?
Adds a description on how to run locally installed npm scripts via npm-run.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

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