docs: add section on keymaps in configuring.md

docs/manual/configuring/keymaps.md

@@ -0,0 +1,35 @@
+# Custom keymaps {#ch-keymaps}
+
+Some plugin modules provide keymap options for convenience. If a keymap is not
+provided by such options, you can easily add custom keymaps yourself via
+`vim.keymaps`:
+
+```nix
+{...}: {
+  config.vim.keymaps = [
+    {
+      key = "<leader>m";
+      mode = "n";
+      silent = true;
+      action = ":make<CR>";
+    }
+    {
+      key = "<leader>l";
+      mode = ["n" "x"];
+      silent = true;
+      action = "<cmd>cnext<CR>";
+    }
+    {
+      key = "<leader>k";
+      mode = ["n" "x"];
+      lua = true;
+      action = ''
+        function()
+          require('foo').do_thing()
+          print('did thing')
+        end
+      '';
+    }
+  ];
+}
+```