[Feature] Document unit convention (pixels v microns) clearly

**Is your feature request related to a problem? Please describe.**
https://forum.image.sc/t/brainmapper-points-transformation-issue/105261/6

**Describe the solution you'd like**
In particular, it's confusing that brainrender and brainrender-napari have differing conventions (because we want tools within napari to be self-consistent) - so this should be very clearly documented. And the fact that our "Export to brainrender" button does the scaling for you.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by reading the linked forum discussion and locating the relevant documentation in the brainglobe.github.io repository. Document the differing pixel and micron conventions between brainrender and brainrender-napari, and explain that the “Export to brainrender” button performs the scaling; done means users can find and understand these rules.

Written by the indexing model from the issue text.