docs(static[logo+favicon]) Add logos and favicons

[tony]

agentgrep's `docs/_static/` was empty (only `.gitkeep`), so the docs site shipped without visual identity. This PR brings agentgrep to parity with sibling tony/* docs sites (`django-docutils`, `django-slugify-processor`) by adding the same logo + favicon + PWA icon set those repos ship.

Summary

• Add `docs/_static/favicon.ico` and the full `docs/_static/img/icons/` tree (logo SVG light/dark, PNG fallback, PWA icon family across Android/Apple/Microsoft platforms, Safari pinned-tab vector).
• Add `docs/manifest.json` — PWA manifest customized for agentgrep (`name`, `description`, `Scope: https://agentgrep.org/\`, `start_url`; `android-chrome-*.png` icon references match the copied tree).
• Wire `docs/conf.py`'s `merge_sphinx_config(...)` with `light_logo`, `dark_logo`, `html_favicon`, `html_extra_path=["manifest.json"]` — matching the `django-docutils/docs/conf.py` precedent.

Asset source

Copied verbatim from `~/work/python/django-docutils/docs/_static/` (byte-identical to `django-slugify-processor`; either source works). The PNG icons cover sizes 36/48/72/96/144/192/256/384/512 for Android Chrome plus Apple touch icons, Microsoft tiles, and a Safari pinned-tab SVG.

Test plan

• `uv run ruff check . --fix --show-fixes` — clean
• `uv run ruff format .` — 11 files unchanged
• `uv run ty check` — clean
• `uv run py.test --reruns 0 -vvv` — 107 passed
• `just build-docs` — build succeeded; rendered HTML includes the sidebar logo, favicon link, and `manifest.json` at the build root
• Post-merge: confirm https://agentgrep.org/ renders the logo in the sidebar and the browser tab carries the favicon