docs: add dev server basic information

docs/pages/04-dev-server.md

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+# Development Server
+
+The tool comes with it's own dev server (since v0.2.4), and can be used to run a
+simple file server for the output folder.
+
+It's as simple as running the following command
+
+```sh
+$ alvu --serve
+# short for
+$ alvu --serve --port=3000
+```
+
+## Live Reload
+
+<small>Added in `v0.2.9`</small>
+
+There's no changes that you need to do, just upgrade your installation to
+`v0.2.9`
+
+#### What's to be expected
+
+- Will reload on changes from the directories `pages`, `public` , or if you
+  changed them with flags then the respective paths will be watched instead
+
+- The rebuilding process is atomic and will recompile a singular file if that's
+  all that's changed instead of compiling the whole folder. This is only true
+  for files in the `pages` directory, if any changes were made in `public`
+  directory then the whole alvu setup will rebuild itself again.
+
+#### Caveats
+
+- `./hooks` are not watched, this is because hooks have their own state and
+  involve a VM, the current structure of alvu stops us from doing this but
+  should be done soon without increasing complexity or size of alvu itself.