Do not generate reference docs for include-files

Prior to this commit, the Asciidoctor Gradle tasks generated top-level
HTML and PDF documents for AsciiDoc files that are included in other
top-level documents. This causes slower builds and results in each
include-file being published twice:

1) inline in the including document (as intended)
2) as a top-level document but missing surrounding context (unintended)

The reason these include-files are generated as top-level documents is
that the asciidoctor and asciidoctorPdf Gradle tasks are configured to
use '*.adoc' as the input source files.

This commit addresses this issue by moving the following include-files
to new subdirectories. Locating the include-files in the subdirectories
causes them to be ignored in the '*.adoc' pattern used to identify
input source files.

- data-access-appendix.adoc -> data-access/data-access-appendix.adoc
- integration-appendix.adoc -> integration/integration-appendix.adoc
- testing-webtestclient.adoc -> testing/testing-webtestclient.adoc

Closes gh-25783

src/docs/asciidoc/data-access/data-access-appendix.adoc

@@ -0,0 +1,91 @@
+= Appendix
+
+
+
+
+[[xsd-schemas]]
+== XML Schemas
+
+This part of the appendix lists XML schemas for data access, including the following:
+
+* <<xsd-schemas-tx>>
+* <<xsd-schemas-jdbc>>
+
+
+
+[[xsd-schemas-tx]]
+=== The `tx` Schema
+
+The `tx` tags deal with configuring all of those beans in Spring's comprehensive support
+for transactions. These tags are covered in the chapter entitled
+<<data-access.adoc#transaction, Transaction Management>>.
+
+TIP: We strongly encourage you to look at the `'spring-tx.xsd'` file that ships with the
+Spring distribution. This file contains the XML Schema for Spring's transaction
+configuration and covers all of the various elements in the `tx` namespace, including
+attribute defaults and similar information. This file is documented inline, and, thus,
+the information is not repeated here in the interests of adhering to the DRY (Don't
+Repeat Yourself) principle.
+
+In the interest of completeness, to use the elements in the `tx` schema, you need to have
+the following preamble at the top of your Spring XML configuration file. The text in the
+following snippet references the correct schema so that the tags in the `tx` namespace
+are available to you:
+
+[source,xml,indent=0]
+[subs="verbatim,quotes"]
+----
+	<?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:aop="http://www.springframework.org/schema/aop"
+		xmlns:tx="http://www.springframework.org/schema/tx" <1>
+		xsi:schemaLocation="
+			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
+			http://www.springframework.org/schema/tx https://www.springframework.org/schema/tx/spring-tx.xsd <2>
+			http://www.springframework.org/schema/aop https://www.springframework.org/schema/aop/spring-aop.xsd">
+
+		<!-- bean definitions here -->
+
+	</beans>
+----
+<1> Declare usage of the `tx` namespace.
+<2> Specify the location (with other schema locations).
+
+NOTE: Often, when you use the elements in the `tx` namespace, you are also using the
+elements from the `aop` namespace (since the declarative transaction support in Spring is
+implemented by using AOP). The preceding XML snippet contains the relevant lines needed
+to reference the `aop` schema so that the elements in the `aop` namespace are available
+to you.
+
+
+
+[[xsd-schemas-jdbc]]
+=== The `jdbc` Schema
+
+The `jdbc` elements let you quickly configure an embedded database or initialize an
+existing data source. These elements are documented in
+<<data-access.adoc#jdbc-embedded-database-support, Embedded Database Support>> and
+<<data-access.adoc#jdbc-initializing-datasource, Initializing a DataSource>>, respectively.
+
+To use the elements in the `jdbc` schema, you need to have the following preamble at the
+top of your Spring XML configuration file. The text in the following snippet references
+the correct schema so that the elements in the `jdbc` namespace are available to you:
+
+[source,xml,indent=0]
+[subs="verbatim,quotes"]
+----
+	<?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:jdbc="http://www.springframework.org/schema/jdbc" <1>
+		xsi:schemaLocation="
+			http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
+			http://www.springframework.org/schema/jdbc https://www.springframework.org/schema/jdbc/spring-jdbc.xsd"> <2>
+
+		<!-- bean definitions here -->
+
+	</beans>
+----
+<1> Declare usage of the `jdbc` namespace.
+<2> Specify the location (with other schema locations).