separate repo for documentation of different versions

[sanbrock]

Currently, no documentation for older releases are available. They could be made available as they can be referenced.

[prjemian]

Legacy docs are bundled up and stored in this repository as agreed previously.

Until we initiate the publishing of versioned documentation, I suggest we continue with the previous agreement. Once we start versioned documentation, we can choose how to handle older docs that do not include version links. Such as point to the repo for legacy content.

[prjemian]

Python, for example, provides a drop-down menu to select other versions of its documentation:

[phyy-nx]

Goals:

• Versioned, published, html docs, including the latest, unreleased docs
• Reduce repo size by moving pdfs in legacy_docs to zenodo. Could involve a gulp force push

Action items:

• How many old releases can we actually build? @PeterC-DLS
• How to set up the docs with a version dropdown? @tacaswell

[phyy-nx]

FYI theme selected in #1472 might impact the version dropdown