Update webpack options for wp-scripts in README.md

[juanmaguitar]

What?

Add output-path flag reference in documentation for build and start commands of wp-scripts

Why?

Because there is no clear reference in the docs about this option for these commands

[github-actions]

Warning: Type of PR label mismatch

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[github-actions]

Warning: Type of PR label mismatch

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Type-related labels to choose from: [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core.
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[github-actions]

Flaky tests detected in f8c4325.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6926744022
📝 Reported issues:

• [Flaky Test] should move to the start of the first line on ArrowUp (-firefox) #56087 in /test/e2e/specs/editor/various/writing-flow.spec.js

[t-hamano]

LGTM! I'm surprised this option hasn't been mentioned explicitly until now 😅