Add support for "warnings as errors"

[Lukasa]

Feature Name

Warnings as errors

Description

Adopters can pass a flag that will promote warnings when building the documentation into errors that fail the doc build.

Motivation

Lots of projects want to build their documentation in CI. In these cases it's possible to introduce docs warnings that provide useful signal, such as notifying us that there is a broken reference in the docs. Catching this in CI allows us to produce the best documentation possible.

Importance

This is fairly important: while it can be retroactively handled by processing the textual output from the build, this is brittle and adds a tricky dependency on the text output format.

Alternatives Considered

No response