[pubrules] add rule to make sure all the resources under the same directory

Dear Editors,

As indicated in the pubrules rules[1], If the document is compound (i.e., if it consists of more than one file), all the files must be under a directory /TR/YYYY/WD-shortname-YYYYMMDD/.

Currently pubrules isn’t checking this, but will apply the rule on April 1st. Every resources that are not in the same directory as the spec is will trigger an error, and block publication. So please make sure the resources (images, stylesheets, scripts, svgs…) are in the right place. 

E.g.
    - spec doc: https://www.w3.org/TR/2021/WD-pubrules-20210401/
    - image (pass): https://www.w3.org/TR/2021/WD-pubrules-20210401/images/sample.png
    - image (pass): https://www.w3.org/TR/2021/WD-pubrules-20210401/sample.png
    - image (error): https://w3c.github.io/pubrules/sample.png

Exceptions are resources under:
    - https://www.w3.org/StyleSheets/TR/
    - https://www.w3.org/scripts/TR/

[1] https://www.w3.org/pubrules/doc/rules/?profile=FPWD#compoundFilesLocation
--
Jiaying Liang
Web Developer, W3C/systeam
+86-182-1069-8997