CI: add an xml formatting check for docs files

[jrfnl]

See the conversation here: #2334 (comment)

Basically we'd need a little (bash or otherwise) script to gather the files in the WordPress/Docs directory and then run the following command against each file with XMLLINT_INDENT set to (four) spaces.

diff -B --tabsize=4 ./WordPress/Docs/Cat/SniffNameStandard.xml <(xmllint --format "./WordPress/Docs/Cat/SniffNameStandard.xml")