Clarify the current migration process for the plugin architecture

[ffjlabo]

What this PR does:

Clarify the current migration process idea for the plugin architecture.

Why we need it:

We want to consider the migration cost as less as possible.

Which issue(s) this PR fixes:

Part of #5252

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):

[t-kikuc]

It's clearer, thank you.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 25.57%. Comparing base (90b5d07) to head (aabd6d6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5368      +/-   ##
==========================================
- Coverage   25.57%   25.57%   -0.01%     
==========================================
  Files         447      447              
  Lines       48004    48004              
==========================================
- Hits        12279    12277       -2     
- Misses      34765    34768       +3     
+ Partials      960      959       -1     

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[khanhtc1202]

LGTM 👍