pw_env_setup: Add doxygen to pigweed's pigweed configuration

Add doxygen as a part of pigweed third party tooling
Doxygen is the de facto standard tool for generating documentation from
annotated C++ sources.
It is desirable to include doxygen binary in the tooling for API
reference doc and for future integration with pw_docgen.

Change-Id: I64460282dd4e57997af18f039ad1b24accf43664
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98340
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>

pw_env_setup/py/pw_env_setup/cipd_setup/pigweed.json

@@ -51,6 +51,16 @@
       ],
       "version_file": ".versions/bloaty-embedded.cipd_version"
     },
+    {
+      "path": "pigweed/third_party/doxygen/${platform}",
+      "platforms": [
+        "linux-amd64"
+      ],
+      "tags":[
+        "version:1.9.4"
+      ],
+      "version_file": ".versions/doxygen.cipd_version"
+    },
     {
       "path": "infra/3pp/tools/protoc/${platform}",
       "platforms": [