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 #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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Warashi]

I think it's nice for users to show kubectl port-forward command and URL of the Web UI.
WDYT?

[ffjlabo]

@Warashi
It's a nice idea. But I wonder if I should do so 🤔
For me, this doc is like a memo to describe whats the manifest is.
Also there is the actual process of the quickstart on the docs. https://pipecd.dev/docs-v0.48.x/quickstart/

[Warashi]

OK, I got it. I agree that we have other documents about these manifests and don't have to show detailed commands here.

[Warashi]

LGTM

[khanhtc1202]

Thank you 👍