Docs: Add configuration.md explaining the theme's parameters (#17)

docs/configuration.md

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+# Configuration
+
+## Theme parameters
+
+The following settings can go either directly into a separate `params.yml` file or into a section `params` in your `config.yml`, e.g.
+
+```yaml
+theme: 'readable'
+title: 'Your Title'
+[...]
+params:
+    theme_parameter: value
+```
+
+### author
+
+Your name.
+
+```yaml
+author: Your name goes here
+```
+
+### mastodon
+
+The link to your Mastodon profile.
+
+```yaml
+mastodon: https://mastodon.example/@profile
+```
+
+### subtitle
+
+The subtitle for your page.
+
+```yaml
+subtitle: This is the subtitle of the website.
+```
+
+### navbar_style
+
+This selects the look of the navigation bar at the top of the page. See the [readable.css wiki](https://codeberg.org/Freedom-to-Write/readable.css/wiki/Navbar-Styles) for possible styles.
+
+> Attention!!!  
+> readable.css changed it's default navbar style from 'classy' to 'default' with v1.1.0.  
+> This theme however chose to keep 'classy' as the default style, so the look of existing websites wouldn't change suddenly after an update.
+
+```yaml
+navbar_style: classy
+```