CI: rfl: build the documentation

Since the `rfl` CI job has not had almost any issue for some weeks,
it is a good time to try to increase a bit the scope of what it tests.

The kernel does not use any particular `rustdoc` unstable issue (apart
from the doctests ones) so far, so in principle it should not introduce
extra issues here, and may be a good extra test case for Rust.

In addition, it may help to test new unstable features in the future.

In the worst case, we can revert it.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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