docs: Add basic documentation system based on nmd

You can now run `nix build '.#docs'` to build HTML documentation
documenting every single option on nixvim! Fortunately, thanks to
the 'description' field, most options are already documented, but
there are still a fair few that need documenting.

I will be taking care of those in the next few days. When those are
done, I will find a way to automatically rebuild documentation on every
repo push, and also add a PR hook requiring documentation.

Additionally, I will try to find a way to have per-page plugin docs.

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1638986258,
-        "narHash": "sha256-OceRdctKZRSgqQxVRvvNB0MaEnFMzQqjUffecoDE9eI=",
+        "lastModified": 1641710811,
+        "narHash": "sha256-yVJ+CtwWZY8BnkNIJ/ue5a28yrRM6CkDF1LvmGmqqwM=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "581d2d6c9cd5c289002203581d8aa0861963a933",
+        "rev": "0ecf7d414811f831060cf55707c374d54fbb1dec",
         "type": "github"
       },
       "original": {
@@ -16,9 +16,26 @@
         "type": "github"
       }
     },
+    "nmdSrc": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1637239786,
+        "narHash": "sha256-l2KsnY537mz0blZdqALZKrWXn9PD39CpvotgPnxyIP4=",
+        "owner": "rycee",
+        "repo": "nmd",
+        "rev": "527245ff605bde88c2dd2ddae21c6479bb7cf8aa",
+        "type": "gitlab"
+      },
+      "original": {
+        "owner": "rycee",
+        "repo": "nmd",
+        "type": "gitlab"
+      }
+    },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "nmdSrc": "nmdSrc"
       }
     }
   },