doc: add CLI help doc to notation key, cert, and notation plugin

[FeynmanZhou]

fix https://github.com/notaryproject/notation/issues/383 Signed-off-by: Feynman Zhou [email protected]

[codecov-commenter]

Codecov Report

Merging #394 (68a743f) into main (53052a9) will decrease coverage by 0.45%. The diff coverage is 42.42%.

@@            Coverage Diff             @@
##             main     #394      +/-   ##
==========================================
- Coverage   32.82%   32.36%   -0.46%     
==========================================
  Files          25       25              
  Lines        1237     1338     +101     
==========================================
+ Hits          406      433      +27     
- Misses        819      893      +74     
  Partials       12       12              

Impacted Files	Coverage Δ
cmd/notation/cert/add.go	29.09% <ø> (ø)
cmd/notation/cert/delete.go	33.33% <ø> (ø)
cmd/notation/cert/generateTest.go	16.52% <ø> (-1.41%)	:arrow_down:
cmd/notation/cert/show.go	30.76% <ø> (ø)
cmd/notation/key.go	23.14% <0.00%> (-1.37%)	:arrow_down:
cmd/notation/plugin.go	0.00% <0.00%> (ø)
cmd/notation/cert/list.go	30.13% <100.00%> (+16.34%)	:arrow_up:
cmd/notation/list.go	23.28% <0.00%> (-0.66%)	:arrow_down:
... and 6 more

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[dtzar]

Once you fix merge conflicts @FeynmanZhou - I think we can go ahead and merge.

[FeynmanZhou]

Looks like this PR is outdated(we have moved to trust-store and trust-polic) and needs to be overhauled.

Yes. I will update it to be consistent with RC.1.

[FeynmanZhou]

@priteshbandi @patrickzheng200 @yizha1 Thanks for your review. I updated it according to your suggestions above and replied to some changes.