fix(docs): Fix docs build during releases (AztecProtocol#3815)

Fixes the issue introduced in AztecProtocol#3762, which caused the docs build step to
always attempt to fetch the last released version. Due to how
release-plase works, the last released version during a release is the
version being released, which is not available as a tag to be fetched.
To avoid this issue, we first check if the tag to be checked out is
available, and if not, just load the code snippets from master.

Author: spalladino

.circleci/config.yml

@@ -892,11 +892,14 @@ jobs:
       - run:
           name: "Require released code"
           command: |
-            echo Requiring released code snippets
-            echo "INCLUDE_RELEASED_CODE=1" >> docs/.env
             LAST_TAG="aztec-packages-v$(jq -r '.["."]' .release-please-manifest.json)"
-            echo Fetching latest released tag $LAST_TAG
-            git fetch origin --refetch --no-filter refs/tags/$LAST_TAG:refs/tags/$LAST_TAG
+            if git ls-remote --tags origin | grep "$LAST_TAG" > /dev/null; then
+              echo "Using code released from $LAST_TAG"
+              echo "INCLUDE_RELEASED_CODE=1" >> docs/.env
+              git fetch origin --refetch --no-filter refs/tags/$LAST_TAG:refs/tags/$LAST_TAG
+            else
+              echo "Skipping as $LAST_TAG is not yet published"
+            fi
       - run:
           name: "Build docs"
           command: build docs