Documentation (#7103)

* added note on importance of site_url

this was in response to
#4678 (comment)

* re-wording to say why site_url is always needed

docs/creating-your-site.md

@@ -37,13 +37,22 @@ This will create the following structure:
 
 ### Minimal configuration
 
-Simply add the following lines to `mkdocs.yml` to enable the theme:
+Simply set the `site_name` and add the following lines to `mkdocs.yml` to enable the theme:
 
-``` yaml
+``` yaml hl_lines="2-5"
+site_name: My site
+site_url: https://mydomain.org/mysite
 theme:
   name: material
 ```
 
+The `site_url` setting is important for a number of reasons.
+By default, MkDocs will assume that your site is hosted at the root of
+your domain. This is not the case, for example, when [publishing to GitHub
+pages] - unless you use a custom domain. Another reason is that some of the
+plugins require the `site_url` to be set, so you should always do this.
+
+  [publishing to GitHub pages]: publishing-your-site.md#github-pages
   [installation methods]: getting-started.md#installation
 
 ???+ tip "Recommended: [configuration validation and auto-complete]"