Documentation improvements

Clean up and improve the documentation following some recent changes.
Specificaly, ensure we consistantly document *two* required settings.

docs/getting-started.md

@@ -65,6 +65,7 @@ Now try editing the configuration file: `mkdocs.yml`. Change the
 
 ```yaml
 site_name: MkLorum
+site_url: https://example.com/
 ```
 
 Your browser should immediately reload, and you'll see your new site name take
@@ -72,6 +73,15 @@ effect.
 
 ![The site_name setting](img/site-name.png)
 
+!!! note
+
+    The [`site_name`][site_name] and [`site_url`][site_url] configuration
+    options are the only two required options in your configuration file. When
+    you create a new project, the `site_url` option is assigned the placeholder
+    value: `https://example.com`. If the final location is known, you can change
+    the setting now to point to it. Or you may choose to leave it alone for now.
+    Just be sure to edit it before you deploy your site to a production server.
+
 ## Adding pages
 
 Now add a second page to your documentation:
@@ -87,6 +97,7 @@ setting:
 
 ```yaml
 site_name: MkLorum
+site_url: https://example.com/
 nav:
     - Home: index.md
     - About: about.md
@@ -114,6 +125,7 @@ changing the theme. Edit the `mkdocs.yml` file and add a [`theme`][theme] settin
 
 ```yaml
 site_name: MkLorum
+site_url: https://example.com/
 nav:
     - Home: index.md
     - About: about.md
@@ -205,5 +217,6 @@ To get help with MkDocs, please use the [GitHub discussions] or [GitHub issues].
 [GitHub discussions]: https://github.com/mkdocs/mkdocs/discussions
 [GitHub issues]: https://github.com/mkdocs/mkdocs/issues
 [site_name]: user-guide/configuration.md#site_name
+[site_url]: user-guide/configuration.md#site_url
 [theme]: user-guide/configuration.md#theme
 [User Guide]: user-guide/index.md