chore: update Stagehand logos and favicon to new branding (browserbase#1967)

## Summary
- Replace old yellow hand icon logos with new "S" logomark across all
locations (README, docs site logos, SVGs)
- Update favicon from old yellow hand to new green "S" logomark
- 10 files updated; no markdown changes needed since filenames were
preserved

## Test plan
- [ ] Verify README logo renders correctly on GitHub (light and dark
mode)
- [ ] Verify docs site logo renders at stagehand.dev
- [ ] Verify favicon appears correctly in browser tab

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Updated Stagehand branding across docs and assets: replaced the yellow
hand with the green “S” logomark, refreshed dark/light wordmark SVGs,
updated favicons, and set the docs accent color to `#00C851`. Filenames
are unchanged; `packages/docs/logo/light.svg` now uses the new wordmark,
and the docs name is set to “Stagehand”.

- **Migration**
  - Verify README logos (light/dark) render on GitHub.
- Verify docs header logos (light/dark) and `/favicon.svg` on
stagehand.dev.
  - Confirm docs UI accents use `#00C851`.
  - Hard refresh or clear cache to see the new favicon and styles.

<sup>Written for commit 8ebc28c.
Summary will update on new commits. <a
href="https://cubic.dev/pr/browserbase/stagehand/pull/1967">Review in
cubic</a></sup>

<!-- End of auto-generated description by cubic. -->

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: shrey150

media/dark_logo.png

@@ -1,11 +1,11 @@
 {
   "$schema": "https://mintlify.com/docs.json",
   "theme": "maple",
-  "name": "🤘 Stagehand",
+  "name": "Stagehand",
   "colors": {
-    "primary": "#B88100",
-    "light": "#FFC83C",
-    "dark": "#FFC83C"
+    "primary": "#00C851",
+    "light": "#00C851",
+    "dark": "#00C851"
   },
   "favicon": "/images/favicon.svg",
   "seo": {