[Feature] Better documentation for `brainglobe_space`

**Is your feature request related to a problem? Please describe.**
@sfmig @viktorpm and I needed to spend quite a while to understand basic brainglobe-space functionality (in particular, the `AnatomicalSpace` constructor and its `map_points_to` function - orientations are tricky. For example, it's not clear from the docstrings or the documentation here, whether and how scale is handled.

**Describe the solution you'd like**
* Examples in the `brainglobe-space` docs showing how re-orienting and scaling are expected to work, and how to instantiate `AnatomicalSpace`s for this.
* As a consequence, a clarification about in what units and orientation `map_points_to` expect its `points` argument.
* in particular, what scale will a target space have when I call `my_anatomical_space.map_points_to("lpi", points)`?
* An explanation/pointing out of the decorator `@to_target` in the dev docs
* updating of existing docs to include the deprecated use of `SpaceConvention` and use `AnatomicalSpace` instead.

**Describe alternatives you've considered**

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**Additional context**
Confusion here arising from trying to[ fix the `admba` atlas.](https://github.com/brainglobe/brainglobe-atlasapi/pull/322)