Add PageDrop to provide Liquid templates with data

docs/_docs/pages.md

@@ -34,3 +34,38 @@ If you have a lot of pages, you can organize them into subfolders. The same subf
 ## Changing the output URL
 
 You might want to have a particular folder structure for your source files that changes for the built site. With [permalinks](/docs/permalinks) you have full control of the output URL.
+
+## Liquid Representation
+
+From Jekyll 4.1 onwards, there is a minor change in how instances of `Jekyll::Page` are exposed to layouts and other Liquid
+templates. `Jekyll::Page` instances now use a `Liquid::Drop` instead of a `Hash`. This change results in greater performance
+for a site with numerous *standlone pages not within a collection*.
+
+### For plugin developers
+
+While end-users need not take any extra action due to this change, plugin authors depending on the existing behavior *may* need
+to make minor changes to their plugins.
+
+If a `Jekyll::Page` subclass' `to_liquid` method calls `super`, it will have to be slightly modified.
+```ruby
+class Foo::BarPage < Jekyll::Page
+  def to_liquid(*)
+    payload = super        # This needs to be changed to `super.to_h`
+                           # to obtain a Hash as in v4.0.0.
+
+    do_something(payload)  # Logic specific to `Foo::BarPage` objects
+  end
+end
+```
+
+`Jekyll::Page` subclasses won't inherit the optimization automatically until the next major version. However, plugin authors
+can opt-in to the optimization in their subclasses by wrapping the temporary `liquid_drop` method if the subclass doesn't
+override the superclass method:
+```ruby
+class Foo::BarPage < Jekyll::Page
+  def to_liquid(*)
+    liquid_drop    # Returns an instance of `Jekyll::Drops::PageDrop`.
+                   # Will be removed in Jekyll 5.0.
+  end
+end
+```