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.

plugins/git/gitgutter.nix

@@ -91,7 +91,7 @@ in {
       };
 
       grep = mkOption {
-        type = types.nullOr (types.oneOf [ types.submodule {
+        type = types.nullOr (types.oneOf [ (types.submodule {
           options = {
             command = mkOption {
               type = types.str;
@@ -103,7 +103,7 @@ in {
               description = "The package of the grep alternative to use";
             };
           };
-        } types.str]);
+        }) types.str]);
         default = null;
         description = "A non-standard grep to use instead of the default";
       };

plugins/statuslines/airline.nix

@@ -30,12 +30,14 @@ in {
       sections = mkOption {
         default = null;
         type = with types; nullOr (submodule {
-          a = sectionOption;
-          b = sectionOption;
-          c = sectionOption;
-          x = sectionOption;
-          y = sectionOption;
-          z = sectionOption;
+          options = {
+            a = sectionOption;
+            b = sectionOption;
+            c = sectionOption;
+            x = sectionOption;
+            y = sectionOption;
+            z = sectionOption;
+          };
         });
       };
 

plugins/statuslines/lightline.nix

@@ -21,7 +21,7 @@ in {
         description = ''
           A list of function component definitions.
 
-          You should define the functions themselves in <para>extraConfig</para>
+          You should define the functions themselves in extraConfig
         '';
         example = ''
           programs.nixvim.plugins.lightline = {

plugins/utils/comment-nvim.nix

@@ -64,8 +64,7 @@ in
           options = {
             basic = mkOption {
               type = types.bool;
-              description = "operator-pending mapping. Includes 'gcc', 'gcb', 'gc[count]{motion}' 
-              and 'gb[count]{motion}'";
+              description = "operator-pending mapping. Includes 'gcc', 'gcb', 'gc[count]{motion}' and 'gb[count]{motion}'";
               default = true;
             };
             extra = mkOption {
@@ -75,8 +74,7 @@ in
             };
             extended = mkOption {
               type = types.bool;
-              description = "extended mapping. Includes 'g>', 'g<', 'g>[count]{motion}' and 
-              'g<[count]{motion}'";
+              description = "extended mapping. Includes 'g&gt;', 'g&lt;', 'g&gt;[count]{motion}' and 'g&lt;[count]{motion}'";
               default = false;
             };
           };

plugins/utils/floaterm.nix

@@ -23,12 +23,12 @@ in
       };
       winWidth = mkOption {
         type = types.nullOr types.float;
-        description = "number of columns relative to &columns.";
+        description = "number of columns relative to &columns.";
         default = null;
       };
       winHeight = mkOption {
         type = types.nullOr types.float;
-        description = "number of lines relative to &lines.";
+        description = "number of lines relative to &lines.";
         default = null;
       };
       borderChars = mkOption {
@@ -38,7 +38,7 @@ in
       };
       rootMarkers = mkOption {
         type = types.nullOr (types.listOf types.str);
-        description = "Markers used to detect the project root directory for --cwd=<root>";
+        description = "Markers used to detect the project root directory for --cwd=&lt;root&gt;";
         default = null;
       };
       opener = mkOption {

plugins/utils/startify.nix

@@ -17,23 +17,25 @@ mkPlugin args {
     lists = mkDefaultOpt {
       description = "Startify display lists. If it's a string, it'll be interpreted as literal lua code";
       global = "startify_lists";
-      type = types.listOf (types.oneOf [types.submodule {
-        type = mkOption {
-          type = types.str;
-          description = "The type of the list";
+      type = types.listOf (types.oneOf [(types.submodule {
+        options = {
+          type = mkOption {
+            type = types.str;
+            description = "The type of the list";
+          };
+          # TODO the header should be a literal lua string!
+          header = mkOption {
+            type = types.nullOr (types.listOf types.str);
+            description = "Optional header. It's a list of strings";
+            default = null;
+          };
+          indices = mkOption {
+            type = types.nullOr (types.listOf types.str);
+            description = "Optional indices for the current list";
+            default = null;
+          };
         };
-        # TODO the header should be a literal lua string!
-        header = mkOption {
-          type = types.nullOr (types.listOf types.str);
-          description = "Optional header. It's a list of strings";
-          default = null;
-        };
-        indices = mkOption {
-          type = types.nullOr (types.listOf types.str);
-          description = "Optional indices for the current list";
-          default = null;
-        };
-      } types.str]);
+      }) types.str]);
 
       value = val: let
         list = map (v: if builtins.isAttrs v then toLuaObject v else v) val;
@@ -131,7 +133,7 @@ mkPlugin args {
     };
 
     enableSpecial = mkDefaultOpt {
-      description = "Show <empty buffer> and <quit>";
+      description = "Show &lt;empty buffer&gt; and &lt;quit&gt;";
       global = "startify_enable_special";
       type = types.bool;
     };