add `treefmt` and `pre-commit-hooks`

I can now run `nix fmt` or `nix flake check` to check the syntax for
various files in this repository.

Fixed a deprecation warning for `hugo`:
```
> WARN  The author key in site configuration is deprecated. Use params.author.email instead.
> WARN  The author key in site configuration is deprecated. Use params.author.name instead.
```

1 files changed, 8 insertions, 0 deletions

diff --git a/treefmt.nix b/treefmt.nix
new file mode 100644
index 0000000..e6164a0
--- /dev/null
+++ b/treefmt.nix
@@ -0,0 +1,8 @@
+{
+  projectRootFile = "flake.nix";
+  programs = {
+    nixfmt.enable = true; # nix
+    taplo.enable = true; # toml
+    yamlfmt.enable = true; # yaml
+  };
+}