feat(core): enable port configuration via environment variable

[OzakIOne]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.

Motivation

Fix #8681

Test Plan

Local test

Test links

Deploy preview: https://deploy-preview-9610--docusaurus-2.netlify.app/

Related issues/PRs

#8681

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 66	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 65	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 74	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 74	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report

[Josh-Cena]

Could you use "Fixes #8681" if your PR fixes that issue? This links the PR to the issue and auto-closes the latter.