Split extensions ref doc page into page with includes.

Closes spring-projects#3243

src/main/antora/modules/ROOT/pages/repositories/core-extensions-populators.adoc

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+[[core.repository-populators]]
+= Repository Populators
+
+If you work with the Spring JDBC module, you are probably familiar with the support for populating a `DataSource` with SQL scripts.
+A similar abstraction is available on the repositories level, although it does not use SQL as the data definition language because it must be store-independent.
+Thus, the populators support XML (through Spring's OXM abstraction) and JSON (through Jackson) to define data with which to populate the repositories.
+
+Assume you have a file called `data.json` with the following content:
+
+.Data defined in JSON
+[source,javascript]
+----
+[ { "_class" : "com.acme.Person",
+ "firstname" : "Dave",
+  "lastname" : "Matthews" },
+  { "_class" : "com.acme.Person",
+ "firstname" : "Carter",
+  "lastname" : "Beauford" } ]
+----
+
+You can populate your repositories by using the populator elements of the repository namespace provided in Spring Data Commons.
+To populate the preceding data to your `PersonRepository`, declare a populator similar to the following:
+
+.Declaring a Jackson repository populator
+[source,xml]
+----
+<?xml version="1.0" encoding="UTF-8"?>
+<beans xmlns="http://www.springframework.org/schema/beans"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xmlns:repository="http://www.springframework.org/schema/data/repository"
+  xsi:schemaLocation="http://www.springframework.org/schema/beans
+    https://www.springframework.org/schema/beans/spring-beans.xsd
+    http://www.springframework.org/schema/data/repository
+    https://www.springframework.org/schema/data/repository/spring-repository.xsd">
+
+  <repository:jackson2-populator locations="classpath:data.json" />
+
+</beans>
+----
+
+The preceding declaration causes the `data.json` file to be read and deserialized by a Jackson `ObjectMapper`.
+
+The type to which the JSON object is unmarshalled is determined by inspecting the `_class` attribute of the JSON document.
+The infrastructure eventually selects the appropriate repository to handle the object that was deserialized.
+
+To instead use XML to define the data the repositories should be populated with, you can use the `unmarshaller-populator` element.
+You configure it to use one of the XML marshaller options available in Spring OXM.
+See the {spring-framework-docs}/data-access/oxm.html[Spring reference documentation] for details.
+The following example shows how to unmarshall a repository populator with JAXB:
+
+.Declaring an unmarshalling repository populator (using JAXB)
+[source,xml]
+----
+<?xml version="1.0" encoding="UTF-8"?>
+<beans xmlns="http://www.springframework.org/schema/beans"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xmlns:repository="http://www.springframework.org/schema/data/repository"
+  xmlns:oxm="http://www.springframework.org/schema/oxm"
+  xsi:schemaLocation="http://www.springframework.org/schema/beans
+    https://www.springframework.org/schema/beans/spring-beans.xsd
+    http://www.springframework.org/schema/data/repository
+    https://www.springframework.org/schema/data/repository/spring-repository.xsd
+    http://www.springframework.org/schema/oxm
+    https://www.springframework.org/schema/oxm/spring-oxm.xsd">
+
+  <repository:unmarshaller-populator locations="classpath:data.json"
+    unmarshaller-ref="unmarshaller" />
+
+  <oxm:jaxb2-marshaller contextPath="com.acme" />
+
+</beans>
+----

src/main/antora/modules/ROOT/pages/repositories/core-extensions-querydsl.adoc

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+[[core.extensions.querydsl]]
+= Querydsl Extension
+
+http://www.querydsl.com/[Querydsl] is a framework that enables the construction of statically typed SQL-like queries through its fluent API.
+
+NOTE: Querydsl maintenance has slowed down to a point where the community has forked the project under OpenFeign at https://github.com/OpenFeign/querydsl (groupId `io.github.openfeign.querydsl`).
+Spring Data supports the fork on a best-effort basis.
+
+Several Spring Data modules offer integration with Querydsl through `QuerydslPredicateExecutor`, as the following example shows:
+
+.QuerydslPredicateExecutor interface
+[source,java]
+----
+public interface QuerydslPredicateExecutor<T> {
+
+  Optional<T> findById(Predicate predicate);  <1>
+
+  Iterable<T> findAll(Predicate predicate);   <2>
+
+  long count(Predicate predicate);            <3>
+
+  boolean exists(Predicate predicate);        <4>
+
+  // … more functionality omitted.
+}
+----
+
+<1> Finds and returns a single entity matching the `Predicate`.
+<2> Finds and returns all entities matching the `Predicate`.
+<3> Returns the number of entities matching the `Predicate`.
+<4> Returns whether an entity that matches the `Predicate` exists.
+
+To use the Querydsl support, extend `QuerydslPredicateExecutor` on your repository interface, as the following example shows:
+
+.Querydsl integration on repositories
+[source,java]
+----
+interface UserRepository extends CrudRepository<User, Long>, QuerydslPredicateExecutor<User> {
+}
+----
+
+The preceding example lets you write type-safe queries by using Querydsl `Predicate` instances, as the following example shows:
+
+[source,java]
+----
+Predicate predicate = user.firstname.equalsIgnoreCase("dave")
+	.and(user.lastname.startsWithIgnoreCase("mathews"));
+
+userRepository.findAll(predicate);
+----