v3 docs do not link to equivalent v4 docs

[BryanCrotaz]

https://wixtoolset.org/docs/v3/howtos/general/specifying_source_files/

This is really useful information but at the top it says it's out of date and you should read the v4 docs instead. However it links only to the root page, and I've gone through the tree and failed to find anything similar in v4.

Some possibilities:

• Nothing has changed: Copy the page to the v4 docs and deep link to it from v3 (which is where google takes you)
• It's subtly changed: Copy the page to the v4 docs and fix the now wrong information, and deep link to it from v3
• It's completely different: Create a new page in v4 and change the note in v3 to say that the mechanism has changed significantly in v4 and you should definitely read the v4 version and ignore this page.