docs: add Building guide (#94999)
Adds a **Building** guide (`docs/01-app/02-guides/building.mdx`): what
`next build` does, how to read the route table (`○` Static, `◐` Partial
Prerender, `ƒ` Dynamic), prerendering dynamic routes with
`generateStaticParams`, streaming request-time work, and debugging
prerender errors. Also links the guide from the deploying page and the
`next build` CLI reference.
