Add template files for consistency.

docs/snippets/type_syntax/_template.md

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+<!--
+This is the template file for a syntax snippet of a Textual CSS type.
+The comments below illustrate some common patterns.
+-->
+
+<!--
+For a simple type like <integer>:
+
+Describe the type in a short paragraph with an absolute link to the type page.
+E.g., “The [`<my-type>`](/css_types/my_type) type is such and such with sprinkles on top.”
+-->
+
+<!--
+For a type with many different values like <color>:
+
+Introduce the type with a link to [`<my-type>`](/css_types/my_type).
+Then, a bullet list with the variants accepted:
+
+ - you can create this type with X Y Z;
+ - you can also do A B C; and
+ - also use D E F.
+
+Consider ending with a link to the code/API that serves as ground truth for what values are accepted.
+-->
+
+<!--
+For a type that accepts specific options like <border>:
+
+Add a sentence and a table. Consider ordering values in alphabetical order if there is no other obvious ordering. See below:
+
+The [`<my-type>`](/css_types/my_type) type can take any of the following values:
+
+| Value         | Description                                   |
+|---------------|-----------------------------------------------|
+| `abc`         | Describe here.                                |
+| `other val`   | Describe this one also.                       |
+| `value three` | Please use full stops.                        |
+| `zyx`         | Describe the value without assuming any rule. |
+-->