doc: Include function doc generation in Makefile

Since #53055 was merged the Makefile for the manual could not be run
correctly as the generated function documentation was included, but
not actually generated.

This adds the necessary generation step by first building the XML file
containing function locations and preserving its store path in a
variable, which is then used both for linking of the locations file
and as a build input for the function docs generator.

This fixes #55014