[Docs] Snippet adapter could build one snippet's source once

[Reinmar]

Some snippets (like docs/_snippets/build-classic-source) are reused in many guides.

Right now, the source of this snippet is build for every guide it's used in. This means a lot of wasted time. This, by side effect, also means many Webpack processes writing to the same file at the same time.

The right way to build them will be to reuse already built (or already being built) snippets.

[Reinmar]

Closed with c13e080.