core: static sites serve a nested directory's index.html

`try_files {path} {path}/index.html /index.html` for SPA sites and the
dashboard: a URL like /docs/setup/ serves that directory's own index.html
before falling back to the root one, so a multi-page static site works at its
sub-paths while single-page apps still get client-side routing.

Found uncommitted in the checkout, author unknown, and already live: wildpc
runs from this tree, so every Caddyfile generated since used it.

core/tests/test_caddyfile.py

@@ -145,7 +145,7 @@ class TestAcmeMode:
         cf = generate_caddyfile_from_registry(_acme({}))
         assert "@host_wildpc host wildpc.example.com" in cf
         assert "root * /data/repos/wildpc/app/dist" in cf
-        assert "try_files {path} /index.html" in cf
+        assert "try_files {path} {path}/index.html /index.html" in cf
         assert "file_server" in cf