docs: doc page for Custom Actions and skaffold exec

[renzodavid9]

Fixes: #8523

Description
This PR adds the new documentation page for Custom Actions + the needed examples.

[codecov]

Codecov Report

Merging #8809 (470ba8f) into main (290280e) will decrease coverage by 6.55%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #8809      +/-   ##
==========================================
- Coverage   70.48%   63.94%   -6.55%     
==========================================
  Files         515      620     +105     
  Lines       23150    31455    +8305     
==========================================
+ Hits        16317    20113    +3796     
- Misses       5776     9837    +4061     
- Partials     1057     1505     +448     

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 410 files with indirect coverage changes

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[container-tools-bot]

Error creating deployment docs-controller-deployment-8809, please visit https://storage.googleapis.com/webhook-logs/logs-8809-1684868732994571668 to view logs.

[container-tools-bot]

Error creating deployment docs-controller-deployment-8809, please visit https://storage.googleapis.com/webhook-logs/logs-8809-1684951843832885133 to view logs.

[container-tools-bot]

Error creating deployment docs-controller-deployment-8809, please visit https://storage.googleapis.com/webhook-logs/logs-8809-1684952516859669471 to view logs.

[aaron-prindle]

LGTM!