Add online documentation build/deployment to conda Jenkins pipeline.

[thomashampson]

Add steps to the deployment pipeline to publish the online user documentation (https://docs.mantidproject.org).

The built HTML is stored in either:

• https://github.com/mantidproject/docs-nightly: The nightly version at https://docs.mantidproject.org/nightly
• https://github.com/mantidproject/docs-versioned: A version published for each release version: https://docs.mantidproject.org/vX.Y.Z

A step needs to be added to the build/deployment pipeline to build the HTML documentation and then publish it to the relevant repository.

Requirements:

• Separate build & publishing steps so that build_packages_with_branch still builds the docs (to check them) but if publishing is disabled the output is just archived
• Similarly to package-conda and package-standalone a script should encapsulate the steps required to build the documentation that can be run by a developer locally.
• Use the following documentation options in CMake:
  • DOCS_HTML=ON
  • DOCS_DOTDIAGRAMS=ON
  • DOCS_SCREENSHOTS=ON
  • DOCS_MATH_EXT="sphinx.ext.mathjax" (Use javascript for equations. Reduces the size and speeds up the build)
  • DOCS_PLOTDIRECTIVE=ON

Nice to have:

• Avoid a full build Mantid again when building the documentation. Can we leverage the fact that we will have built the Conda packages and just install them to built the HTML?

For reference, the old non-conda, non-pipeline HTML docs were built by this script.