Custom contracts docs

[nguyer]

Resolves #398

[codecov-commenter]

Codecov Report

Merging #604 (7550206) into main (3d0ab63) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              main      #604    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          306       311     +5     
  Lines        18185     18784   +599     
==========================================
+ Hits         18185     18784   +599     

Impacted Files	Coverage Δ
pkg/fftypes/pin.go	100.00% <0.00%> (ø)
pkg/fftypes/data.go	100.00% <0.00%> (ø)
pkg/fftypes/batch.go	100.00% <0.00%> (ø)
pkg/fftypes/event.go	100.00% <0.00%> (ø)
pkg/fftypes/namespace.go	100.00% <0.00%> (ø)
pkg/fftypes/operation.go	100.00% <0.00%> (ø)
pkg/fftypes/tokenpool.go	100.00% <0.00%> (ø)
internal/config/config.go	100.00% <0.00%> (ø)
internal/data/blobstore.go	100.00% <0.00%> (ø)
internal/metrics/metrics.go	100.00% <0.00%> (ø)
... and 50 more

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[nickgaski]

Super stuff with the smart contract interfaces and straightforward events. Thanks @nguyer