fix(perf): serve favicon and logo locally instead of remote Zenodo URLs

[rain-Brian]

What

Vendors the site favicon/logo locally instead of loading them from a remote Zenodo URL on every page.

• Adds docs/assets/cat.png (the existing brand icon, copied from the Zenodo record).
• mkdocs.yml: favicon/logo → assets/cat.png (was https://zenodo.org/records/15376499/files/cat.png).

Why

Every page currently makes a cross-origin request to zenodo.org for the favicon and logo — a render-blocking external dependency and a measurable performance hit (Lighthouse Performance is 70). Serving them locally removes two external requests sitewide and is the cheapest available perf win.

Verification

mkdocs build --strict clean; built homepage contains no zenodo.org references; site/assets/cat.png resolves under /MegaDetector/.

[zhmiao]

Why are we changing the link from remote to local?

[rain-Brian]

Why are we changing the link from remote to local?

Two reasons: (1) it removes a third-party dependency — every page was fetching the favicon + logo from zenodo.org, so the site degrades if Zenodo is slow or down; (2) it let me optimize the asset. The Zenodo file was a 301KB, 415×416 PNG used for both the favicon and the header logo, which only ever render at 16–48px. I vendored the same image locally and downsized it to 192×192 / 17KB (94% smaller, transparency preserved, visually identical at display size). Net: one fewer cross-origin request and ~285KB off every page. It won't single-handedly move the Lighthouse score (LCP/FCP dominate the current 70), but it's a clean, safe win and removes the external dependency.

[zhmiao]

@rain-Brian , there is a conflict in this pr