docs: add a Migrating from Bun guide

[bartlomieju]

The migrate section's direction is "migrating TO Deno from anywhere", but it
only covered Node. This adds /runtime/migrate/migrate_from_bun/ for the Bun
audience: run an existing project (leading with the permission-prompt
difference, the one thing Bun users won't expect), a verified command
cheatsheet, API equivalents for the big surfaces (Bun.serve to Deno.serve,
bun:sqlite to node:sqlite, Bun.file, bun:test, Bun.$), and an honest section
on what has no direct equivalent (macros, HTMLRewriter).

Every disputed fact was checked rather than assumed: node:sqlite confirmed
by importing it, HTMLRewriter confirmed absent the same way, deno bundle
footnoted as experimental per its own page, Bun-side claims verified against
bun.com/docs, and bunx maps to deno x (per our own CLI reference) rather
than a deno run incantation. The page is deliberately not yet linked from
the migrate hub or sidebar; that wiring lands with tomorrow's sidebar
regroup to avoid conflicting with the PRs already in flight.