Document sinogram geometries

[JeffFessler]

No description provided.

[codecov]

Codecov Report

Merging #17 (d124d29) into main (1e43b8a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          444       448    +4     
=========================================
+ Hits           444       448    +4     

Impacted Files	Coverage Δ
src/Sinograms.jl	100.00% <100.00%> (ø)
src/fbp2/sino-geom.jl	100.00% <100.00%> (ø)
src/fbp2/sino-plot.jl	100.00% <100.00%> (ø)
src/units.jl	100.00% <100.00%> (ø)

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