docs: specify site-url

Fixes 404 page's relative links and styling being relative to `/`.

docs/mdbook/book.toml

@@ -5,6 +5,9 @@ multilingual = false
 src = "."
 title = "nixvim docs"
 
+[output.html]
+site-url = "@SITE_URL@"
+
 [output.html.fold]
 enable = true
 level = 0

docs/mdbook/default.nix

@@ -321,6 +321,10 @@ pkgs.stdenv.mkDerivation (finalAttrs: {
       cp "$file" "$path"
     done
 
+    # Patch book.toml
+    substituteInPlace ./book.toml \
+      --replace-fail "@SITE_URL@" "$siteURL"
+
     # Patch SUMMARY.md - which defiens mdBook's table of contents
     substituteInPlace ./SUMMARY.md \
       --replace-fail "@PLATFORM_OPTIONS@" "$wrapperOptionsSummary" \
@@ -332,6 +336,10 @@ pkgs.stdenv.mkDerivation (finalAttrs: {
     cp -r ${search}/* $dest/search
   '';
 
+  # The root directory of the site
+  # Can be overridden, e.g. by CI
+  siteURL = "/nixvim/";
+
   inherit (mdbook)
     nixvimOptionsSummary
     wrapperOptionsSummary