Document that extensions should be stateless

The "Registering Extensions" page did not mention how often an extension is
instantiated or that a single instance is typically reused across tests. Add a
short section clarifying that no assumptions should be made about extension
instantiation, that extensions should be stateless, and that state should be
kept in the ExtensionContext.Store, linking to "Keeping State in Extensions".

Author: JonnasFigueiredo

documentation/modules/ROOT/pages/extensions/registering-extensions.adoc

@@ -492,3 +492,13 @@ programmatically via `@RegisterExtension` or `@ExtendWith` on fields in a subcla
 NOTE: A specific extension implementation can only be registered once for a given
 extension context and its parent contexts. Consequently, any attempt to register a
 duplicate extension implementation will be ignored.
+
+[[registration-instances]]
+== Extension Instances and State
+
+You should not make any assumptions about when or how often the JUnit Platform instantiates
+a registered extension. In particular, a single extension instance is typically reused
+across multiple tests, so an extension should be _stateless_. Any state that needs to be
+kept between invocations should be stored in the `{ExtensionContext_Store}` rather than in
+instance fields, so that it is scoped correctly and does not leak between tests. See
+xref:extensions/keeping-state-in-extensions.adoc[Keeping State in Extensions] for details.