Don't physically copy static files for mkdocs serve

[oprypin]

Just read them from their original location in the live server instead

• Closes no copy serve for large sites(10Gb) #2662

[squidfunk]

Have you tested compatibility with plugins that augment the serve capabilities of MkDocs? For example, the mkdocs monorepo plugin provides a consolidated serve mode, same as the projects plugin.

Edit: so, reading the source of this PR, I think it should work without problems, but I think it might be a good idea to test the changes downstream, as build is part of the public API and the changes are quite substantial ☺️