feat(neovim): configure markdownlint

Prevent diagnostic messages for duplicate sibling headings and line
length.

config/neovim/lua/opdavies/lsp/none-ls.lua

@@ -29,7 +29,7 @@ null_ls.setup {
       end,
     },
     diagnostics.markdownlint.with {
-      extra_args = { "--disable", "MD013" },
+      extra_args = { "--config", "~/.markdownlint.yaml" },
     },
     diagnostics.php,
     diagnostics.phpcs.with {

system/shared/modules/neovim.nix

@@ -243,4 +243,13 @@ in
       shellcheck
     ];
   };
+
+  home.file.".markdownlint.yaml".text = ''
+    default: true
+
+    line-length: false
+
+    no-duplicate-heading:
+      siblings_only: true
+  '';
 }