Improve doc building process

[ianthomas23]

When you build the docs locally or via CI, it rebuilds many old versions of the docs as well as the current ones. This isn't ideal as it takes a long time to do so, and uploading to the doc site uses a lot of bandwidth. Also, we should not recreate and upload old versions of the docs, they should remain as they were when released - the rebuilding may be using different versions of dependencies that causes the output to render differently.

So the proposal is to change the doc build system to only build a specific version (e.g. the latest tagged release, or current git HEAD). The hosting situation for the docs, and the automatic upload system also need to be considered so that they all continue to work together.

[ianthomas23]

There is probably some issue about links between the different versions of the docs in the HTML pages, so this needs to be considered too.