Consistently prefer "Refaster rule" over "Refaster template" (#286)

As the former term references a class containing one or more `@BeforeTemplate`
methods, one or more `@Placeholder` methods and an optional `@AfterTemplate`, 
while the latter term more narrowly references a single `@BeforeTemplate` or 
`@AfterTemplate` method.

website/_config.yml

@@ -4,7 +4,7 @@ logo: assets/images/favicon.svg
 url: https://error-prone.picnic.tech
 description: >-
   Error Prone extensions: extra bug checkers and a large battery of Refaster
-  templates.
+  rules.
 
 theme: just-the-docs
 plugins:

website/refasterrules.md

@@ -1,8 +1,8 @@
 ---
 layout: default
-title: Refaster templates
+title: Refaster Rules
 nav_order: 2
 has_children: true
 ---
 
-# Refaster templates
+# Refaster Rules