chore: remove legacy Zola site and superseded files

The Astro app in apps/site is now the production build (root Dockerfile
already builds it, not `zola build`), so the old Zola templates, Sass,
static assets, and build tooling are dead weight:

- templates/, sass/, static/, themes/, zola.toml, Makefile, bin/update-zola
- content/posts/ (fully migrated to content/writings/) and the legacy
  content/archive, content/about.md, content/_index.md pages
- legacy OG-image/llms.txt generation scripts (superseded by the
  *.astro-site.mjs versions apps/site's build already uses)
- unrelated stray files (manimations/, build.sh)

Also drops apps/site/src/pages/search.astro, replaced by the search modal,
and swaps in the real favicon.ico.

.vscode/settings.json

@@ -1,3 +0,0 @@
-{
-    "snyk.advanced.autoSelectOrganization": true
-}