Add reference pipelines file for Gerrit driver

As it has been done for the Pagure driver, include
a reference pipelines section in the Gerrit driver
documentation. It helps the reader to better
understand the big picture of the Gerrit's pipelines
configuration but also provide an almost ready to use
pipelines definition file.

Change-Id: I5234476f7393ba938e8365b8c908a2ff6f9e8c76

doc/source/admin/drivers/gerrit.rst

@@ -333,3 +333,11 @@ order to be enqueued into the pipeline.
            my-gerrit:
              approval:
                - Code-Review: [-1, -2]
+
+Reference pipelines configuration
+---------------------------------
+
+Here is an example of standard pipelines you may want to define:
+
+.. literalinclude:: ../examples/zuul-config/zuul.d/gerrit-reference-pipelines.yaml
+   :language: yaml

doc/source/admin/examples/zuul-config/zuul.d/gerrit-reference-pipelines.yaml

@@ -0,0 +1,132 @@
+- pipeline:
+    name: check
+    description: |
+      Newly uploaded patchsets enter this pipeline to receive an
+      initial +/-1 Verified vote.
+    manager: independent
+    precedence: low
+    require:
+      gerrit:
+        open: True
+        current-patchset: True
+    trigger:
+      gerrit:
+        - event: patchset-created
+        - event: change-restored
+        - event: comment-added
+          comment: (?i)^(Patch Set [0-9]+:)?( [\w\\+-]*)*(\n\n)?\s*recheck
+    success:
+      gerrit:
+        # Note that gerrit keywords are case-sensitive.
+        Verified: 1
+      sqlreporter:
+    failure:
+      gerrit:
+        Verified: -1
+      sqlreporter:
+
+- pipeline:
+    name: gate
+    description: |
+      Changes that have been approved by core reviewers are enqueued
+      in order in this pipeline, and if they pass tests, will be
+      merged. For documentation on how gating with Zuul works, please see
+      https://zuul-ci.org/docs/zuul/user/gating.html
+    manager: dependent
+    precedence: normal
+    supercedes: check
+    post-review: True
+    require:
+      gerrit:
+        open: True
+        current-patchset: True
+        approval:
+          - Workflow: 1
+    trigger:
+      gerrit:
+        - event: comment-added
+          approval:
+            - Workflow: 1
+    start:
+      gerrit:
+        Verified: 0
+    success:
+      gerrit:
+        Verified: 2
+        submit: true
+      sqlreporter:
+    failure:
+      gerrit:
+        Verified: -2
+      sqlreporter:
+    window-floor: 20
+    window-increase-factor: 2
+
+- pipeline:
+    name: post
+    description: |
+      This pipeline runs jobs that operate after each change is
+      merged. Queue items are identified by the abbreviated hash (git
+      log --format=%h) of the merge commit.
+    manager: supercedent
+    precedence: high
+    post-review: True
+    trigger:
+      gerrit:
+        - event: ref-updated
+          ref: ^refs/heads/.*$
+    success:
+      sqlreporter:
+    failure:
+      sqlreporter:
+
+- pipeline:
+    name: promote
+    description: |
+      This pipeline runs jobs that operate after each change is merged
+      in order to promote artifacts generated in the gate
+      pipeline.
+    manager: supercedent
+    precedence: high
+    post-review: True
+    trigger:
+      gerrit:
+        - event: change-merged
+    success:
+      gerrit: {}
+      sqlreporter:
+    failure:
+      gerrit: {}
+      sqlreporter:
+
+- pipeline:
+    name: release
+    description: |
+      When a commit is tagged as a release, this pipeline runs jobs
+      that publish archives and documentation.
+    manager: independent
+    precedence: high
+    post-review: True
+    trigger:
+      gerrit:
+        - event: ref-updated
+          ref: ^refs/tags/[0-9]+(\.[0-9]+)*$
+    success:
+      sqlreporter:
+    failure:
+      sqlreporter:
+
+- pipeline:
+    name: tag
+    post-review: true
+    description: This pipeline runs jobs in response to any tag event.
+    manager: independent
+    precedence: high
+    trigger:
+      gerrit:
+        - event: ref-updated
+          ref: ^refs/tags/.*$
+    success:
+      sqlreporter:
+    failure:
+      sqlreporter: