chore: add mdbook in flake (#1299)

* nix-flake: use follows for `rust-overlay` (..)

This removes the extra nixpkgs dependency by re-using the nixpkgs
already defined.

https://nixos.wiki/wiki/Flakes
https://web.archive.org/web/20230621091703/https://nixos.wiki/wiki/Flakes

> # The `follows` keyword in inputs is used for inheritance.
> # Here, `inputs.nixpkgs` of sops-nix is kept consistent with the `inputs.nixpkgs` of
> # the current flake, to avoid problems caused by different versions of nixpkgs.

* nix-flake: use nix overlays for rustc/cargo (..)

This allows the same nightly rust version to be used across the
flake (vs. strictly in the `devShell`).

* nix-flake: add `mdbook` to the development shell (..)

bumped `nixpkgs` to the latest unstable to pull in mdbook version `0.4.30`.

* nix-flake: expose all rust tools in dev environment (..)

The `oxalica / rust-overlay` docs expose all tools in the development
environment, so we should do the same:

https://github.com/oxalica/rust-overlay#use-in-devshell-for-nix-develop

---------

Co-authored-by: Jay Querie <jay@querie.cc>

flake.lock

@@ -38,11 +38,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1681920287,
-        "narHash": "sha256-+/d6XQQfhhXVfqfLROJoqj3TuG38CAeoT6jO1g9r1k0=",
+        "lastModified": 1687898314,
+        "narHash": "sha256-B4BHon3uMXQw8ZdbwxRK1BmxVOGBV4viipKpGaIlGwk=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "645bc49f34fa8eff95479f0345ff57e55b53437e",
+        "rev": "e18dc963075ed115afb3e312b64643bf8fd4b474",
         "type": "github"
       },
       "original": {
@@ -52,22 +52,6 @@
         "type": "github"
       }
     },
-    "nixpkgs_2": {
-      "locked": {
-        "lastModified": 1681358109,
-        "narHash": "sha256-eKyxW4OohHQx9Urxi7TQlFBTDWII+F+x2hklDOQPB50=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "96ba1c52e54e74c3197f4d43026b3f3d92e83ff9",
-        "type": "github"
-      },
-      "original": {
-        "owner": "NixOS",
-        "ref": "nixpkgs-unstable",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
@@ -78,7 +62,9 @@
     "rust-overlay": {
       "inputs": {
         "flake-utils": "flake-utils_2",
-        "nixpkgs": "nixpkgs_2"
+        "nixpkgs": [
+          "nixpkgs"
+        ]
       },
       "locked": {
         "lastModified": 1682043560,