feat: automated Lighthouse CI audits for pull requests

[shivambisen]

Summary of Changes

This pull request introduces a new, automated Lighthouse CI workflow that runs on every pull request. The workflow helps contributors and reviewers monitor website performance, accessibility, SEO, and best practices on a per-page basis, preventing performance regressions before merging into the main codebase.

The workflow is configurable ( it will only run when explicitly requested pages with trigger word)

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How to Use This Feature

To trigger a Lighthouse CI audit:

1. Add the phrase Lighthouse Test to your PR body.
2. Include a list of the pages you want to audit, each on a separate line.

Example PR Body:

This PR updates the adopters and blog pages.

Lighthouse Test
/
/adopters/
/blog/
/get-involved/

---

Workflow Behavior

• If Lighthouse Test is present in the PR body:

  • Runs a full Gatsby build once.
  • Starts a local static server.
  • Runs Lighthouse audits only on the specified paths.
  • Generates a detailed Lighthouse report with a link provided in the workflow logs.

• If Lighthouse Test is not present:

  • Lighthouse-specific steps are skipped.
  • Workflow completes very quickly (5-6 seconds)

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closes #716

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[shivambisen]

@xmulligan @paularah , could you please review this when you have a moment?

[paularah]

Hi @shivambisen, for a feature like this, you should get the conversation rolling first in the issue, before opening a PR.

This is a nice to have feature but I am not sure we absolutely need this. I'll wait to hear from other people.