Add reference to Subfiles documentation for \subfix in Wiki

[N9199]

Preliminary questions [Required]

Disable all the other extensions except for LaTeX Workshop, restart VS Code, and check that you can not see the requested feature. [Required]

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Make sure to visit the wiki FAQ before requesting a feature.

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Is your requested feature related to a problem? Please describe. [Required]

No

Screenshots

Reference the above in the 4th point in https://github.com/James-Yu/LaTeX-Workshop/wiki/Compile#the-root-file as it's the official solution given by the subfiles package.