eww: howto: remove nodes with @aria-hidden=true

[T.V Raman]

An EWW feature request:

Web sites that use JS heavily also try to add attribute
"@aria-hidden=true" to nodes that are not relevant to tools like
adaptive technologies -- it's becoming a good tool to nuke crud on web
pages.

At present EWW renders these nodes --- and they mostly add noise --
could we add a customization var to EWW that allows one to junk these nodes?

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