Add docs for quickstart with raw manifests on README

[ffjlabo]

What this PR does / why we need it:

as title

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 22.79%. Comparing base (60283d0) to head (84e8c31). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5152 +/- ## ========================================== - Coverage 22.80% 22.79% -0.02% ========================================== Files 412 412 Lines 43827 43827 ========================================== - Hits 9996 9990 -6 - Misses 33044 33050 +6 Partials 787 787 ```

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