docs: restructure documentation

docs/manual/configuring/custom-plugins/old-method.md

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+# Old Method {#sec-old-method}
+
+Prior to version 0.5, the method of adding new plugins was adding the plugin package to `vim.startPlugins` and add
+its configuration as a DAG under `vim.configRC` or `vim.luaConfigRC`. Users who have not yet updated to 0.5, or prefer
+a more hands-on approach may use the old method where the load order of the plugins is determined by DAGs.
+
+## Adding plugins {#sec-adding-plugins}
+
+To add a plugin to neovim-flake's runtime, you may add it
+
+```nix
+{pkgs, ...}: {
+  # add a package from nixpkgs to startPlugins
+  vim.startPlugins = [
+    pkgs.vimPlugins.aerial-nvim  ];
+}
+```
+
+And to configure the added plugin, you can use the `luaConfigRC` option to provide configuration
+as a DAG using the neovim-flake extended library.
+
+```nix
+{inputs, ...}: let
+  # assuming you have an input called neovim-flake pointing at the neovim-flake repo
+  inherit (inputs.neovim-flake.lib.nvim.dag) entryAnywhere;
+in {
+  vim.luaConfigRC.aerial-nvim= entryAnywhere ''
+    require('aerial').setup {
+      -- your configuration here
+    }
+  '';
+}
+```