Move plugin examples from README to optional plugin files (#831)

* Move autopairs example from README to an optional plugin

* Move neo-tree example from README to an optional plugin

lua/kickstart/plugins/neo-tree.lua

@@ -0,0 +1,25 @@
+-- Neo-tree is a Neovim plugin to browse the file system
+-- https://github.com/nvim-neo-tree/neo-tree.nvim
+
+return {
+  'nvim-neo-tree/neo-tree.nvim',
+  version = '*',
+  dependencies = {
+    'nvim-lua/plenary.nvim',
+    'nvim-tree/nvim-web-devicons', -- not strictly required, but recommended
+    'MunifTanjim/nui.nvim',
+  },
+  cmd = 'Neotree',
+  keys = {
+    { '\\', ':Neotree reveal<CR>', { desc = 'NeoTree reveal' } },
+  },
+  opts = {
+    filesystem = {
+      window = {
+        mappings = {
+          ['\\'] = 'close_window',
+        },
+      },
+    },
+  },
+}