Basic docs boilerplate.
Checkpoint on the Dependency Injection tutorial chapter.

This gets us as far as implementing dependency injection by passing in
dependencies as arguments, but we still need to elaborate on how that
affects the broader program and where tyedie can help with that.
Another DI tutorial checkpoint.

Got to the point where we've described dependencies using tiedye.
However, there seems little point in describing the current low-level
provider interface since we're going to add another layer on top for
simpler declaration of groups of providers and then this part would
need to get rewritten anyway.
Add the concept of a ProviderSet.

ProviderSet is intended as a more convenient way to modularize sets of
providers that are related in some way, as well as to encourage exposing
providers as functions that can be unit tested, rather than burying them
as local lambdas, etc.
Merge branch 'master' into docs
Complete the DI tutorial, referencing the new ProviderSet class.