Added note on sponsors to plugins

docs/plugins/meta.md

@@ -10,6 +10,13 @@ pages in a folder, i.e., a subsection of your project, which is particularly
 useful to ensure that a certain subset of pages features specific tags, uses a
 custom template, or is attributed to an author.
 
+---
+
+<!-- md:sponsors --> __Sponsors only__ – this plugin is currently reserved to
+[our awesome sponsors].
+
+  [our awesome sponsors]: ../insiders/index.md
+
 ## Objective
 
 ### How it works

docs/plugins/optimize.md

@@ -10,7 +10,13 @@ The optimize plugin automatically identifies and optimizes all media files when
 As a result, your site loads significantly faster and yields better rankings in
 search engines.
 
+---
+
+<!-- md:sponsors --> __Sponsors only__ – this plugin is currently reserved to
+[our awesome sponsors].
+
   [building your project]: ../creating-your-site.md#building-your-site
+  [our awesome sponsors]: ../insiders/index.md
 
 ## Objective
 

docs/plugins/privacy.md

@@ -11,6 +11,13 @@ external assets. With just a single line of configuration, the plugin can
 automatically identify and download external assets, making GDPR compliance
 as effortless as it can possibly be.
 
+---
+
+<!-- md:sponsors --> __Sponsors only__ – this plugin is currently reserved to
+[our awesome sponsors].
+
+  [our awesome sponsors]: ../insiders/index.md
+
 ## Objective
 
 ### How it works

docs/plugins/projects.md

@@ -10,6 +10,13 @@ distinct projects, build them concurrently and preview them together as one.
 This is particularly useful when creating a multi-language project, but can also
 be used to split very large projects into smaller parts.
 
+---
+
+<!-- md:sponsors --> __Sponsors only__ – this plugin is currently reserved to
+[our awesome sponsors].
+
+  [our awesome sponsors]: ../insiders/index.md
+
 ## Objective
 
 ### How it works

docs/plugins/typeset.md

@@ -10,6 +10,13 @@ headlines within the navigation and table of contents. This means that code
 blocks, icons, emojis and any other inline formatting can be rendered exactly
 as defined in the page's content.
 
+---
+
+<!-- md:sponsors --> __Sponsors only__ – this plugin is currently reserved to
+[our awesome sponsors].
+
+  [our awesome sponsors]: ../insiders/index.md
+
 ## Objective
 
 ### How it works