docs: add Migrating from Node.js to Deno guide

[bartlomieju]

The "Migrating from Node" content lived as a couple of sections buried
inside the Node and npm compatibility page, and it opened with a list of
caveats that undersold Deno's actual drop-in compatibility — leading with
"expect little to no changes" and three things to watch out for.

This moves it into its own Migrating from Node.js to Deno guide and
rewrites the introduction to lead with the real expectation: most Node
projects run in Deno with no changes at all. The node: prefix, explicit
Buffer import, and CommonJS notes are demoted to the rare exceptions
they are, framed as "if something doesn't run immediately" rather than
required migration steps.

The guide is extracted from node.md (which keeps a short pointer to it),
carries the migration and cheatsheet oldUrl redirects, fixes a few stale
links in the moved content, and updates the inbound link from the web
development page. A sidebar entry is added next to the Node page.

This branches from main and overlaps the runtime IA restructure in #3190
on _data.ts; once #3190 lands I will rebase this and move the sidebar
entry into the new Guides group.