Fix manual Makefile by including function doc generation

[tazjin]

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

Side note: Recreating the derivation dependencies this way in the Makefile is a little bit awkward. Better suggestions welcome! 🤔

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Motivation for this change

Manual builds were broken (as per #55014)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[samueldr]

When reproducing the failure/success, be sure you make clean, as the file isn't re-generated without cleaning.