Clarify public dir in directory structure doc page (getzola#2238) (g…

…etzola#2240)

* Clarify `public` dir (getzola#2238)

* Add `zola serve` to `public` dir description

* Move and rewrite `public` clarification

docs/content/documentation/getting-started/directory-structure.md

@@ -18,6 +18,8 @@ After running `zola init`, you should see the following structure in your direct
 5 directories, 1 file
 ```
 
+You might also see a `public` directory if you are running the default `zola build/serve` commands which contains some output for the site: the full site for `zola build` and only the static assets for `zola serve`. This folder will be deleted/created automatically by `zola serve`.
+
 Here's a high-level overview of each of these directories and `config.toml`.
 
 ## `config.toml`