[Docs] add an example for global config

[tejal29]

Fixes: #4305
A user reported from reading global config docs, it wasnt clear to create or add global config file.
Add a sample global config and specify this gets created when user runs skaffold config set

[tejal29]

Docs changes #4305 (comment)

[container-tools-bot]

Please visit http://35.236.7.240:1313 to view changes to the docs.

[codecov]

Codecov Report

Merging #4341 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4341      +/-   ##
==========================================
- Coverage   71.81%   71.78%   -0.03%     
==========================================
  Files         324      324              
  Lines       12476    12499      +23     
==========================================
+ Hits         8960     8973      +13     
- Misses       2948     2957       +9     
- Partials      568      569       +1     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/render.go	42.85% <0.00%> (-4.77%)	⬇️
cmd/skaffold/app/cmd/deploy.go	68.57% <0.00%> (-0.88%)	⬇️
pkg/skaffold/deploy/helm.go	78.13% <0.00%> (-0.75%)	⬇️
cmd/skaffold/app/cmd/flags.go	86.36% <0.00%> (ø)
pkg/skaffold/config/options.go	100.00% <0.00%> (ø)
pkg/skaffold/deploy/kustomize.go	77.71% <0.00%> (+0.13%)	⬆️
pkg/skaffold/deploy/kubectl.go	67.78% <0.00%> (+0.13%)	⬆️
pkg/skaffold/deploy/labels.go	55.95% <0.00%> (+1.07%)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	63.41% <0.00%> (+2.43%)	⬆️

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