Rename a subdirectory under doc to match what analysis_server has

Change-Id: Ibb4447511570a61b71a8c92cf6d7fe738d6054a6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/256216
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
diff --git a/pkg/analyzer/doc/design/summaries.md b/pkg/analyzer/doc/implementation/summaries.md
similarity index 100%
rename from pkg/analyzer/doc/design/summaries.md
rename to pkg/analyzer/doc/implementation/summaries.md