LaTeX Tip: How to define variable like \subtitle{...} which can be used from the preamble

Quite often when working with LaTeX, it is quite convenient to be able to define things like subtitles and/or other extra fields to be included in the title section, as per the screenshot below:

Now, before we go any further, I should mention that I am aware of the existence of ready-made custom document classes out there in the wild which often provide the necessary bits and pieces already. But, for argument sake, let's say that we want to keep things lightweight (i.e. fewer deps on external packages) or want to know how to do this in future should the need arise.