Avoid resolving content during rendering

[d-ronnqvist]

During a documentation build, DocC resolves all content (both local and external links and assets) before rendering begins. Still, some rendering code resolve content again as a way to access the resolved content.

If we could change these to lookup the resolved content elsewhere then we'd be able to avoid some unneycceassy work of resolving links and content references that are known to fail. We'd also be able to release data held by the link resolver and data providers, lowering memory usage during rendering.