Guide about unit and scale aware rendering

[TimMonko]

References and relevant issues

Closes #38
Closes #954

napari PRs:
the original units to layers PR: napari/napari#6979
rendering calculation with units: napari/napari#7889
updating unit rendering example: napari/napari#8767
setting pixel as default scale bar unit: napari/napari#8900 (comment)
scale bar units from layers: napari/napari#8907 (comment)
removing scale_bar.unit (will be merged soon): napari/napari#9007

napari-metadata discussion: how things work with pint and units: napari/napari-metadata#115

discussions about scale bar, units, appearance, and scale:
https://forum.image.sc/t/increasing-font-size-and-repositioning-the-scale-bar/97972/3
https://forum.image.sc/t/check-out-and-and-give-feedback-on-napari-0-7-0rc0/119662/2

Description

Adding narrative documentation as a guide about scale and unit aware rendering.
See: #release > 0.7.1 units and scalebar -- not working as intended?? for my ask for help and discussion on some issues
I intentionally cross reference to gallery examples and other documentation to avoid duplication and maintenance.

[brisvag]

Left some suggestions!

One general note: while I get the reason of always pairing scale with units (it's a simple hand-in-hand combination), it should be made clear that all transforms are unit-aware (at least I hope so ??)

[TimMonko]

@Czaki, I added highlight to this because I think its worth linking to in the release notes to help explain all the changes that come in 0.7.1 (and also 0.7.0)