mdbook >= 0.5.0 will break the build

[thilobillerbeck]

As experienced while updating the nixos-homepage, mdbook seems to have introduced breaking changes with their 0.5 release that will influence the nix-pills build. This was first discovered here NixOS/nixos-homepage#1932.

While I hotfixed this through building nix-pills with stable nixpkgs for now, it will break again when the 0.5 release propagates through to nixos-25.11.

From what I discovered glancing at this the multilingual = false config option is not supported anymore and should be removed. Additionally there is an issue brought up by the linkchecker:

# using mdbook 0.5.2
$ nix-build -E 'with import <nixpkgs> { }; callPackage ./default.nix { }'
[...]
Caused by:
    missing field `sections` at line 1 column 195289
ERROR Renderer exited with non-zero return code.
ERROR Rendering failed
        Caused by: The "linkcheck" renderer failed
error: builder for '/nix/store/n9g6dypmv0c7bma6cl7ik1g69am226va-nix-pills.drv' failed with exit code 101;
       last 20 log lines:
       > Running phase: unpackPhase
       > unpacking source archive /nix/store/hjf73dfpmgmjj7flyg4a7q5jj17n7mby-nix-pills
       > source root is nix-pills
       > Running phase: patchPhase
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: configurePhase
       > no configure script, doing nothing
       > Running phase: buildPhase
       >  INFO Book building has started
       >  INFO Running the html backend
       >  INFO HTML book written to `/build/nix-pills/book/html`
       >  INFO Running the linkcheck backend
       >  INFO Invoking the "linkcheck" renderer
       > Error: Unable to parse RenderContext
       >
       > Caused by:
       >     missing field `sections` at line 1 column 195289
       > ERROR Renderer exited with non-zero return code.
       > ERROR Rendering failed
       >     Caused by: The "linkcheck" renderer failed
       For full logs, run 'nix log /nix/store/n9g6dypmv0c7bma6cl7ik1g69am226va-nix-pills.drv'.

When I can spare some more time I may look at this myself, but if anyone with some more insights and familiarity with the build process wants to take a look, this would be greatly appreciated. :)