Split the Starlark docs into Build APIs vs not

Summary: Useful to distinguish what is pure Starlark vs our build extensions, and thus what makes sense in targets files vs rules.

Reviewed By: stepancheg

Differential Revision: D44749219

fbshipit-source-id: 974730eafd69096cc0072cbe07a946f5cbb0e4eb

docs.py

@@ -94,8 +94,13 @@ def main() -> None:
                 dest = "rules"
                 prefix += "# Rules\n\nThese rules are available as standard in Buck2.\n"
                 src = "\n".join(src.splitlines()[1:])
+            elif x.endswith("/function.bzl"):
+                # Uninteresting docs we'd rather not have generated
+                continue
+            elif "/standard/" in x or "/extension/" in x or x.endswith("builtins.md"):
+                dest = "starlark/" + name
             else:
-                dest = x[:-3]
+                dest = "build/" + x[7:-3]
             dest = "docs/api/" + dest + ".generated.md"
             os.makedirs(Path(dest).parent, exist_ok=True)
             write_file(dest, prefix + src)

website/sidebars.js

@@ -181,8 +181,13 @@ const manualSidebar = [
       },
       {
         type: 'category',
-        label: 'Rule APIs',
-        items: [{ type: 'autogenerated', dirName: 'api/native' }],
+        label: 'Starlark APIs',
+        items: [{ type: 'autogenerated', dirName: 'api/starlark' }],
+      },
+      {
+        type: 'category',
+        label: 'Build APIs',
+        items: [{ type: 'autogenerated', dirName: 'api/build' }],
       },
       {
         type: 'category',