Add note about custom distDir to standalone build docs (#48592)
### What?
Add a note that if a custom `distDir` has been set, the standalone build
can be found at the specified location and the static files folder has
to be copied to the custom location as well.
### Why?
This may be obvious, but such a note would've saved me some minutes, not
gonna lie.
### How?
\-
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Co-authored-by: Steven <steven@ceriously.com>
