Add option to have a prefix for a taxonomy

As seen in #2449, it might be nice to not serve the taxonomies in the
top level directory, but in a subfolder. This commit adds an option
"prefix" to the taxonomy definition.

The taxonomy then would be served in /<prefix>/<taxonomy_name> instead of
/<taxonomy>.

Author: Mandragorian

Diff for: components/config/src/config/taxonomies.rs

@@ -5,6 +5,8 @@ use serde::{Deserialize, Serialize};
 pub struct TaxonomyConfig {
     /// The name used in the URL, usually the plural
     pub name: String,
+    /// The prefix used in the URL.
+    pub prefix: Option<String>,
     /// The slug according to the config slugification strategy
     pub slug: String,
     /// If this is set, the list of individual taxonomy term page will be paginated
@@ -21,6 +23,7 @@ impl Default for TaxonomyConfig {
     fn default() -> Self {
         Self {
             name: String::new(),
+            prefix: None,
             slug: String::new(),
             paginate_by: None,
             paginate_path: None,

Diff for: components/site/src/lib.rs

@@ -930,6 +930,10 @@ impl Site {
             components.push(taxonomy.lang.as_ref());
         }
 
+        if let Some(prefix) = &taxonomy.kind.prefix {
+            components.push(prefix.as_ref());
+        }
+
         components.push(taxonomy.slug.as_ref());
 
         let list_output =