doc: add more details and examples to import openapi guide

[liuxiran]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

improve IMPORT_OPENAPI_USER_GUIDE.md, add more details and examples to help users to import route from the correct OAS3.0 file.

Related issues

close #1464
close #1660

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-io]

Codecov Report

Merging #1672 (9a618cf) into master (c6c5d14) will decrease coverage by 5.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1672      +/-   ##
==========================================
- Coverage   72.59%   67.00%   -5.59%     
==========================================
  Files         133       47      -86     
  Lines        5728     3116    -2612     
  Branches      666        0     -666     
==========================================
- Hits         4158     2088    -2070     
+ Misses       1327      776     -551     
- Partials      243      252       +9     

Flag	Coverage Δ
backend-e2e-test	47.36% <ø> (-14.77%)	⬇️
backend-e2e-test-ginkgo	?
backend-unit-test	51.77% <ø> (-0.04%)	⬇️
frontend-e2e-test	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...l/handler/route_online_debug/route_online_debug.go	5.55% <0.00%> (-70.38%)	⬇️
api/internal/handler/healthz/healthz.go	66.66% <0.00%> (-33.34%)	⬇️
api/internal/core/entity/entity.go	81.25% <0.00%> (-18.75%)	⬇️
api/internal/filter/schema.go	39.49% <0.00%> (-15.97%)	⬇️
api/internal/core/store/storehub.go	63.55% <0.00%> (-11.22%)	⬇️
api/internal/handler/schema/schema.go	90.47% <0.00%> (-9.53%)	⬇️
api/internal/handler/handler.go	68.51% <0.00%> (-9.26%)	⬇️
api/internal/handler/route/route.go	71.42% <0.00%> (-7.35%)	⬇️
api/internal/handler/ssl/ssl.go	67.53% <0.00%> (-5.76%)	⬇️
api/internal/filter/authentication.go	72.22% <0.00%> (-5.56%)	⬇️
... and 93 more

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[nic-chen]

@liuxiran

thanks so much.

Need to add some instructions on importing duplicate data, as well as instructions on the status of the route after import.

[starsz]

@juzhiyuan @LiteSun Help check the Frontend CI.

[juzhiyuan]

Hi, please first update the markdown

[nic-chen]

LGTM from the content. Thanks @liuxiran

[juzhiyuan]

LGTM

[imjoey]

/lgtm