templates/swh-pipeline: Add stage to build sphinx documentation

If it exists a sphinx tox environment in the built project, it will be executed to build sphinx documentation and check for errors.

Related to T3258

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Migrated from D5617 (view on Phabricator)

jobs/templates/includes/agent-docker-sphinx.groovy.j2

+agent {
+  docker {
+    reuseNode true
+    image 'swh-jenkins/{{sphinx_docker_image}}'
+    args '--mount type=volume,src=shared-jenkins-cachedir,dst=/home/jenkins/.cache'
+  }
+}