docs: document cmd template function

[ericzzzzzzz]

Fixes: #8882

Description

• add a new template functions subtitle
• document cmd template function.

[codecov]

Codecov Report

Merging #8929 (d34007c) into main (290280e) will decrease coverage by 6.86%.
The diff coverage is 49.29%.

@@            Coverage Diff             @@
##             main    #8929      +/-   ##
==========================================
- Coverage   70.48%   63.62%   -6.86%     
==========================================
  Files         515      624     +109     
  Lines       23150    31925    +8775     
==========================================
+ Hits        16317    20312    +3995     
- Misses       5776    10086    +4310     
- Partials     1057     1527     +470     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/completion.go	13.04% <0.00%> (-1.25%)	⬇️
cmd/skaffold/app/cmd/config/list.go	65.21% <ø> (ø)
cmd/skaffold/app/cmd/config/set.go	88.72% <ø> (ø)
cmd/skaffold/app/cmd/config/util.go	54.28% <ø> (ø)
cmd/skaffold/app/cmd/credits.go	100.00% <ø> (ø)
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/generate_pipeline.go	60.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_modules.go	65.00% <ø> (ø)
cmd/skaffold/app/cmd/inspect_profiles.go	66.66% <ø> (ø)
... and 40 more

... and 416 files with indirect coverage changes

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