Migrate site templates to yaml config (#7189)

Does some winter cleaning on the site-template stuff :)

1. Migrated away from the custom conf format to plain old yaml
2. Made the images and image files match with the name from the yaml
file. (It's not a problem to have spaces in file paths, nor urls so I
don't see why we shouldn't)
3. Updated the relevant documentation

There is a small breakage in the api response for the meta field. It now
returns `{"screenshot_dimensions":{"width":1000,"height":800}}` which I
believe is a lot more useful than
`{"meta.screenshot_dimensions":"1000x800"}`

Besides, I don't believe this endpoint has any third-party consumers
that rely on the metadata since it's a rarely known feature anyways

types/features/site-templates.interfaces.ts

@@ -23,10 +23,21 @@
 // POSSIBILITY OF SUCH DAMAGE.
 
 export type SiteTemplatesMetadata = {
-    screenshot_dimensions?: string;
+    screenshot_dimensions: {
+        width: number;
+        height: number;
+    };
 };
 
-export type SiteTemplatesType = {
+export type SiteTemplateConfiguration = {
+    meta: SiteTemplatesMetadata;
+    templates: {
+        name: string;
+        reference: string;
+    }[];
+};
+
+export type SiteTemplateResponse = {
     meta: SiteTemplatesMetadata;
     templates: Record<string, string>;
 };