FIX: update doc EXAMPLE_HEADER to match sphinx-gallery

[rcomer]

PR summary

Followed the instructions at

matplotlib/lib/matplotlib/tests/test_doc.py

Lines 6 to 10 in e253aa2

	We have copied EXAMPLE_HEADER and modified it to include meta keywords.
	This test monitors that the version we have copied is still the same as
	the EXAMPLE_HEADER in sphinx-gallery. If sphinx-gallery changes its
	EXAMPLE_HEADER, this test will start to fail. In that case, please update
	the monkey-patching of EXAMPLE_HEADER in conf.py.

Note that in CI this test only runs under Appveyor because the others do not install sphinx-gallery.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[rcomer]

Ref: sphinx-gallery/sphinx-gallery#1255

[rcomer]

Ooops - just realised this is already covered by #28103.