[Feature] An in-depth "explanation" of our considerations, assumptions and choices in template builder

**Is your feature request related to a problem? Please describe.**
We are working on a HowTo guide (in the diataxis sense) in #457 to cover our template building pipeline.

**Describe the solution you'd like**
An "explanation" in the [diataxis sense](https://diataxis.fr/explanation/) to help people gain in-depth context around our template builder pipelines and how they fit into the wider ecosystem.

They should include (among other things) the "why"-questions raised by @PolarBean in https://github.com/brainglobe/brainglobe.github.io/pull/465

* [two questions about masking](https://github.com/brainglobe/brainglobe.github.io/pull/465/changes#r3250065295)
* in particular how what the masking parameters mean, and approaches to adapting them
* [some design choices about preprocessing](https://github.com/brainglobe/brainglobe.github.io/pull/465/changes#r3250115632)
* [reasoning behind padding volumes](https://github.com/brainglobe/brainglobe.github.io/pull/465/changes#r3250100903)
* [reasoning behind copying txt files manually to their own folders](https://github.com/brainglobe/brainglobe.github.io/pull/465/changes/BASE..ab9f52d424a28898d398629980d01f615d637845#r3260005130)