[docs] Add minimal non-CLI documentation

[twardoch]

The https://font-bakery.readthedocs.io/en/latest/structure is a bit curious.

The first section looks like it's mostly for contributors. Most users simply trust that the software does its advertised job and doesn't necessarily need convincing. Overall, it's wordy and ChatGPT-sounding, like https://font-bakery.readthedocs.io/en/latest/product/Overview.html#what-is-the-opportunity-for-designers-and-foundries-to-collaborate-on-font-quality-assurance

4. Why contribute publicly? Common goals with shared costs

Good quality is a common goal for all designers and foundries, not only those working on libre fonts. Developing good tools to ensure quality is expensive, time consuming, and requires deep expertise. Working in a public participatory fashion, ‘in the open,’ can be a cost-effective business strategy for type designers and font publishers because the costs are shared, and the benefits are mutual. (...)

This would be best moved to the section for contributors.

On the other hand, the user section is missing a large chunk: it teaches you how to use fontbakery CLI but does not mention how to use it as a Python library.

It would be great to have a minimal example there of how to call fontbakery "as a library", from within an existing Python script.

[twardoch]

Or #4397