Stop checking non-asciidoc files for doc snippets (#84390)

When SnippetsTask looks for doc snippets, the list of files it
checks includes roughly 350 files that aren't asciidoc files. Image
files (both png and jpg), yaml files, and so on. Set an explicit
include pattern so that Gradle skips these files instead of trying
to read them.

docs/build.gradle

@@ -15,8 +15,7 @@ apply plugin: 'elasticsearch.docs-test'
 apply plugin: 'elasticsearch.rest-resources'
 
 ext.docsFileTree = fileTree(projectDir) {
-  // No snippets in here!
-  exclude 'build.gradle'
+  include '**/*.asciidoc'
   // That is where the snippets go, not where they come from!
   exclude 'build/**'
   exclude 'build-idea/**'