docs: Update shell autocompletion instructions

[jopit]

The docs currently describe how to enable shell auto completion for the rollouts CLI when run as a standalone binary, but the don't describe how to get auto completion to work when used from kubectl.

This PR updates the docs with instructions on how this can be done, based on the information here: https://github.com/kubernetes/enhancements/tree/master/keps/sig-cli/2379-kubectl-plugins#shell-completion-support

This works with kubectl version 1.26 and newer.

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Go Published Test Results

2 130 tests   2 130 ✅  2m 50s ⏱️
  118 suites      0 💤
    1 files        0 ❌

Results for commit ca248ed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8405f2e) 81.83% compared to head (ca248ed) 83.00%.
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3377      +/-   ##
==========================================
+ Coverage   81.83%   83.00%   +1.16%     
==========================================
  Files         135      135              
  Lines       20688    16873    -3815     
==========================================
- Hits        16931    14005    -2926     
+ Misses       2883     1996     -887     
+ Partials      874      872       -2     

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E2E Tests Published Test Results

  4 files    4 suites   3h 44m 32s ⏱️
107 tests  95 ✅  6 💤  6 ❌
442 runs  404 ✅ 24 💤 14 ❌

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Results for commit ca248ed.

[kzap]

@jopit how does this work just by putting a file in /usr/local/bin/ ? Is that a known path or must one also setup kubectl argo rollouts completion bash ?

[jopit]

@kzap to provide completion for a plugin when run by kubectl, the kubectl command will search the PATH for an executable file using the prefix kubectl_complete- followed by the plugin name. That executable must determine what the appropriate completions are and write them to stdout. More details can be found here: https://github.com/kubernetes/enhancements/tree/master/keps/sig-cli/2379-kubectl-plugins#shell-completion-support

There is no need to setupkubectl argo rollouts completion bash -- that provides completions when the plugin executable is run standalone.