fix(docs): add null examples header and expected failure

[IndexSeek]

Description of changes

Adds the section header for the examples for null.

I added the expected failure annotation, but the # quartodoc: +EXPECTED_FAILURE comment still shows up in the render.

I wasn't sure if there was something special I needed to do for it not to show up, like how this is here but the # doesn't show up in the pivot_longer examples.

ibis/ibis/expr/types/relations.py

Lines 3807 to 3811 in 4d99018

	>>> who.pivot_longer( # quartodoc: +EXPECTED_FAILURE
	... s.index["new_sp_m014":"newrel_f65"],
	... names_to=["diagnosis", "gender", "age"],
	... names_pattern="new_?(.*)_.(.*)",
	... )