Update quickstart docs

[khanhtc1202]

What this PR does / why we need it:

Make quickstart easier with only kubectl command required

Which issue(s) this PR fixes:

Part of https://github.com/pipe-cd/pipecd/issues/5148

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 22.94%. Comparing base (e9dd4c5) to head (887ed43). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5184 +/- ## ========================================== + Coverage 22.90% 22.94% +0.03% ========================================== Files 416 419 +3 Lines 44690 44986 +296 ========================================== + Hits 10238 10322 +84 - Misses 33660 33870 +210 - Partials 792 794 +2 ```

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