Exclude 3rd parth jquery library for builds and policheck

[wadepickett]

Update the aspnetcore/docfx.json file to exclude the jquery-validation library of files.
It should exclude all files that include this path: "/lib/jquery-validation/"

Example:
"exclude": [ "/includes/", "/license.md", "/obj/", "/readme.md", "/sample/", "/samples/", "_site/", "/lib/jquery-validation/" ],

Why:
This is so Policheck does not attempt validate those files, throwing errors on code that cannot be changed on that 3rd party library for the sake of the PR.

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Associated WorkItem - 547136