Document inputDigest tagger, and move sha256 tagger to end

[briandealwis]

Fixes: #5842

Description
Document the new inputDIgest tagger and de-emphasize the sha256 tagger by moving it to the end.

User facing changes (remove if N/A)

• the new inputDigest tagger uses a digest of the source files to the image

[codecov]

Codecov Report

Merging #5948 (4b53c8e) into master (fafdcd7) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5948      +/-   ##
==========================================
- Coverage   70.67%   70.64%   -0.03%     
==========================================
  Files         454      454              
  Lines       17395    17403       +8     
==========================================
+ Hits        12294    12295       +1     
- Misses       4195     4198       +3     
- Partials      906      910       +4     

Impacted Files	Coverage Δ
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
...ffold/kubernetes/portforward/resource_forwarder.go	78.87% <0.00%> (-1.73%)	⬇️
pkg/skaffold/docker/image.go	78.34% <0.00%> (-1.39%)	⬇️
pkg/skaffold/event/v2/event.go	74.03% <0.00%> (+0.12%)	⬆️
pkg/skaffold/runner/v1/dev.go	73.11% <0.00%> (+0.12%)	⬆️
pkg/skaffold/docker/parse.go	87.14% <0.00%> (+0.95%)	⬆️
...skaffold/kubernetes/debugging/container_manager.go	50.98% <0.00%> (+0.98%)	⬆️
...affold/kubernetes/portforward/forwarder_manager.go	50.00% <0.00%> (+1.11%)	⬆️

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