Documentation for Custom Test in Skaffold

[PriyaModali]

Fixes: ##5520
Description
Adding documentation for the newly implemented Custom Test feature.

• Created a new directory for Test
• Migrated existing Test doc page to Container Structure Test and moved it under Test directory
• Added a new doc page for Custom Test with all the relevant content and examples
• Added new samples for Custom Test
• Added references to Test at relevant places in the Skaffold doc site
• Updated all the relevant links and paths to support the newly added content
• Added Test to the info panel on the right
• Added maturity level alpha to the newly added Custom Test feature

User facing changes
Three new pages are added to the Skaffold documentation:

• A new page, http://35.236.47.144:1313/docs/pipeline-stages/testers/, describing the supported Skaffold Tests
• A new page, http://35.236.47.144:1313/docs/pipeline-stages/testers/structure/, describing the Container Structure Tests
• A new page, http://35.236.47.144:1313/docs/pipeline-stages/testers/custom/, describing the Custom Tests

Related:
#5333

[container-tools-bot]

Please visit http://35.236.47.144:1313 to view changes to the docs.

[codecov]

Codecov Report

Merging #5521 (c189b4b) into master (3160270) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5521      +/-   ##
==========================================
+ Coverage   71.32%   71.35%   +0.03%     
==========================================
  Files         400      400              
  Lines       14831    14831              
==========================================
+ Hits        10578    10583       +5     
+ Misses       3475     3473       -2     
+ Partials      778      775       -3     

Impacted Files	Coverage Δ
pkg/skaffold/docker/parse.go	82.16% <0.00%> (-1.09%)	⬇️
pkg/skaffold/docker/image.go	79.53% <0.00%> (+1.39%)	⬆️
pkg/skaffold/util/tar.go	56.00% <0.00%> (+5.33%)	⬆️

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[container-tools-bot]

Please visit http://35.236.47.144:1313 to view changes to the docs.