docs/fix-links: init

The README/CONTRIBUTING files are authored with GitHub in mind, but we
want to re-use them for the docs website.

Replace the existing simple substitution with a pandoc AST-based filter.

docs/fix-links/default.nix

@@ -0,0 +1,33 @@
+{
+  lib,
+  runCommand,
+  pandoc,
+  githubUrl ? "https://github.com/nix-community/nixvim/blob/main/",
+  docsUrl ? "https://nix-community.github.io/nixvim/",
+}:
+src:
+runCommand (src.name or (builtins.baseNameOf src))
+  {
+    inherit src;
+    bindings =
+      lib.generators.toLua
+        {
+          asBindings = true;
+        }
+        {
+          inherit githubUrl docsUrl;
+        };
+    filter = ./filter.lua;
+    nativeBuildInputs = [ pandoc ];
+  }
+  ''
+    echo "$bindings" > filter.lua
+    cat $filter >> filter.lua
+
+    pandoc \
+        --output $out \
+        --from markdown \
+        --to markdown-raw_attribute \
+        --lua-filter filter.lua \
+        $src
+  ''