Include list of Macrostrat timescales in docs

[willgearty]

Sending people to the raw API list seems lazy. I propose two better options:

1. Make the get_scale_data() docs dynamic and download and parse the Macrostrat options when the help page is loaded (this seems complicated and would break anytime there isn't internet or the Macrostrat API is down)
2. Update the docs as part of the data-raw workflow. This would include a date that the list was compiled and then it would add the list of available timescales to the documentation. I think the easiest way to do this would be to create a template file with the list as @details that is then used in get_scale_data.R. This way we don't need to edit the get_scale_data.R file, we only ever regenerate the template file.

[willgearty]

Once this is done, we can then direct people to the get_scale_data docs instead of the Macrostrat API list.