docs: use gray favicon in dev mode

[Saithej2k]

Summary

• Add a gray EUI favicon for local development.
• Use the dev favicon only when the docs site runs with NODE_ENV=development.

Testing

• git diff --check
• prettier checks for docusaurus.config.ts and favicon-dev.svg
• xmllint --noout packages/website/static/favicon-dev.svg
• git push pre-push checks

Closes #9732

[github-actions]

👋 Since this is a community submitted pull request, a Buildkite build has not been started automatically. Would an Elastic organization member please verify the contents of this pull request and kick off a build manually?