chore: mark irrelevant snapshot files as linguist-generated (#27277)

With this, GitHub will not show them by default in diffs. The diff can still be viewed, but only on request.

Tested on this PR: https://github.com/aws/aws-cdk/pull/27278/files

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.gitattributes

@@ -11,3 +11,8 @@
 *.jpg binary
 *.tgz binary
 *.tar.gz binary
+
+# Hide snapshots from GitHub UI, except for the actual templates
+**/*.snapshot/** linguist-generated
+**/*.snapshot/**/*.template.json -linguist-generated
+**/*.snapshot/**/*DeployAssert*.template.json linguist-generated