Allow creating a plot viewing website outside of the workflow

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### What problem does your feature request solve?

CSET offers a powerful diagnostic viewing interface, and its requirement on diagnostics (an index.html and meta.json file in a directory) could also be satisfied with other tools, or when running the CSET command line manually.

We should make is easy for people to take advantage of this plot viewing capability, even when not using the workflow to run CSET. This should also benefit academics and others who may not want to, or be able to, run the cylc workflow.

### Describe the solution you'd like

Merge `install_website_skeleton` and `finish_website` into a single function, and implement as a `cset` CLI command. That way `finish_website` can just call that command as its implementation. This should also make it easier to test.

### Describe alternatives you've considered

We don't _have_ to do this, we could just try and make the workflow easier to run, or perhaps allow sticking in extra non-CSET plots. But doing this doesn't preclude making the workflow easier (and should simplify it by merging the tasks) and I don't see an obvious way for people to add in extra plots into CSET without adding significant complexity for a niche use case.

Locate the implementations and workflow call sites for install_website_skeleton and finish_website, then trace how the CSET CLI commands are defined. Done means a CLI command can create the plot-viewing website outside the workflow, finish_website delegates to the merged implementation, and the workflow continues to work.

Written by the indexing model from the issue text.