Rework targets, introducing `make serve` or `make serve_local`

[carlopi]

make serve will build with the same defaults as shell.duckdb.org, and eventually serve the equivalent content to be opened in a browser tab.

make serve_local will do the same with 2 major changes: unsigned extension are enabled AND custom_extension_directory is made to point to the local one (instead of the remote one).

[Mytherin]

Thanks!