[Docs] Show a gray favicon in dev mode

[MannXo]

Summary

Fixes #9732

packages/website/docusaurus.config.ts hardcoded favicon: 'favicon.ico', so the local dev site shows the same colored favicon as production. With the dev server and eui.elastic.co open in adjacent tabs, it's easy to lose track of which one you're looking at.

This adds a favicon-dev.svg, a desaturated version of the existing EUI mark (static/images/eui_logo.svg) using an SVG feColorMatrix grayscale filter on the original colored paths rather than a single flat gray fill — this preserves the relative luminance/contrast between the four shapes, so the mark stays recognizable at favicon size instead of reading as a flat blob. docusaurus.config.ts now picks favicon-dev.svg when NODE_ENV === 'development' (reusing the same env check already used one line above for storybookBaseUrl), falling back to the existing favicon.ico otherwise.

Note: the reference screenshot originally attached to this issue (user-attachments/assets/47241641-...) is dead (404, confirmed authenticated) — this was independently verified against source and by comparing a flat-gray vs. grayscale-filter render side by side.

API Changes

N/A — docs site theme change only, no @elastic/eui public API affected.

Screenshots

Before (prod, colored)	After (dev, gray)

Functionally verified: yarn build (prod) resolves <link rel="icon" href="/favicon.ico">; yarn start (dev) resolves <link rel="icon" href="/favicon-dev.svg">.

Impact Assessment

• 🔴 Breaking changes
• 💅 Visual changes
• 🧪 Test impact
• 🔧 Hard to integrate

Impact level: 🟢 None — local dev-only, no consumer-facing or production change.

Release Readiness

• Documentation
• Figma
• Migration guide
• Adoption plan

QA instructions for reviewer

• Run the docs site locally (yarn start) and confirm the browser tab shows the gray favicon
• Confirm a production build (yarn build) still resolves the colored favicon.ico

Checklist before marking Ready for Review

• Filled out all sections above
• QA: Tested light/dark modes, high contrast, mobile, Chrome/Safari/Edge/Firefox, keyboard-only, screen reader
• QA: Tested in CodeSandbox and Kibana
• QA: Tested docs changes
• Tests: Added/updated Jest, Cypress, and VRT
• Changelog: Added changelog entry (docs-only change per changelogs.md — requesting skip-changelog label in a comment, no permission to self-apply)
• Breaking changes

Reviewer checklist

• Approved Impact Assessment — Acceptable to merge given the consumer impact.
• Approved Release Readiness — Docs, Figma, and migration info are sufficient to ship.

[MannXo]

This is a documentation-only change to packages/website/, which per changelogs.md doesn't need a changelog entry. Could a maintainer add the skip-changelog label to bypass the CI check?

[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?

[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

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[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: f0ab6ca

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: f0ab6ca
• Documentation website
• Storybook
• 🚫 Visual regression tests skipped (remove the skip-vrt label to re-enable)