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

Additional details and impacted files

@@            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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[Warashi]

LGTM

[t-kikuc]

nits

[t-kikuc]

I think we don't need to mention the default project.
If needed, The PipeCD control plane will be installed with a default project named quickstart. should be moved to the next step.

Suggested change

	The PipeCD control plane will be installed with a default project named `quickstart`. To access the PipeCD Control Plane UI, run the following command
	Run the following command to access the PipeCD Control Plane UI.

[khanhtc1202]

@t-kikuc I would like to keep this as is, since I mentioned the project=quickstart in the below sentence like this 👀

[t-kikuc]

OK,
I meant users don't need to know about the project when executing the portforward.

[ffjlabo]

👌