docs: Improve Quick start instructions (#1458)

* docs: Improve Quick start instructions

- Slightly tweak Python server instructions to be more generic
- Clarify the section about customizing the main container

* Update quickstart.md

Co-authored-by: 沈唁 <52o@qq52o.cn>

Author: waldyrious

docs/quickstart.md

@@ -81,7 +81,7 @@ If you prefer to lock docsify to a specific version, specify the full version af
 
 ### Manually preview your site
 
-If you installed python on your system, you can easily use it to run a static server to preview your site.
+If you have Python installed on your system, you can easily use it to run a static server to preview your site.
 
 ```bash
 cd docs && python -m SimpleHTTPServer 3000
@@ -97,7 +97,7 @@ If you want, you can show a loading dialog before docsify starts to render your
 <div id="app">Please wait...</div>
 ```
 
-You should set the `data-app` attribute if you changed `el`:
+By default, the `id` attribute of the main container is `app`. If you want to use a different id, you must also set the `data-app` attribute, and [configure](configuration.md#el) the `el` parameter accordingly:
 
 ```html
 <!-- index.html -->
@@ -110,5 +110,3 @@ You should set the `data-app` attribute if you changed `el`:
     }
   </script>
 ```
-
-Compare [el configuration](configuration.md#el).