Fail docs build on warnings

[tomazfernandes]

Context

• We recently had a docs formatting regression that would have been caught if the docs build failed on warnings.
• There is already a fail-on-warning configuration, but it is currently disabled/commented out.

Proposal

• Re-enable fail-on-warning for docs.
• Fix the current warnings so CI stays green going forward.

Why

• This prevents formatting regressions from being merged by failing the docs build on warnings.