[Feature] Better documentation for `brainglobe_space`

[alessandrofelder]

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 AnatomicalSpaces 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.