For now, sphinx 6.0 is broken, so we can keep 5.3 until fixing doc
for 6.0

GitLab: #15
Change-Id: I2a708f5218f9566fafd96349c80268ecece5796e

* _build/Dockerfile.build: New Dockerfile for docs-build pipeline,
with the required tools for building the docs installed.
* _build/Jenkinsfile.build: New Jenkinsfile defining the docs-build
pipeline.

Change-Id: If50e687a5b9342eb2dcb5cb39e8a63db91f8616d

* .gitignore: Ignore GNU gettext *.mo binary files.
* Makefile: New tx-pull and tx-push rules for pulling translations
from and pushing translation source strings to Transifex using their
tx tool.
* _build/Dockerfile.i18n: New Dockerfile for docs-i18n pipeline, to
push and pull translation sources/strings on a weekly basis.
* _build/Jenkinsfile.18n: New Jenkinsfile defining the docs-i18n
pipeline.

Change-Id: I945e9d087ef559f18ee1f9afd8f5035d6c35c737