docs: add module setup to gin quickstart

[tatakaisun]

Pull Request Checklist

Please ensure your pull request meets the following requirements:

• [ ×] Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

Thank you for contributing!

Summary

• Add Go module setup steps to the README quickstart.
• Keep the change limited to documentation.

Why

New users following the quickstart from an empty directory need to initialize a Go module and resolve dependencies before running go run main.go.

Changes

• Add go mod init example.com/gin-quickstart.
• Add go mod tidy before running the example.
• Renumber the quickstart steps accordingly.

Testing

• Checked go version: go1.26.3 windows/amd64.
• Reproduced the README quickstart in a clean temporary directory.
• Ran go mod init example.com/gin-quickstart.
• Created main.go from the README example.
• Ran go mod tidy.
• Ran go run main.go.
• Verified curl http://localhost:8080/ping returns {"message":"pong"}.

Known existing failures

go test ./... was run under the gin repository, but failed in existing tests unrelated to this README-only change:

• TestSaveUploadedFileWithPermission
• TestFileDescriptor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (b758ec4).
⚠️ Report is 276 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4657      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3138      -44     
==========================================
- Hits         3157     3087      -70     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <ø> (?)
-tags go_json	98.29% <ø> (?)
-tags nomsgpack	98.35% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <ø> (?)
go-1.26	98.37% <ø> (?)
macos-latest	98.37% <ø> (-0.84%)	⬇️
ubuntu-latest	98.37% <ø> (-0.84%)	⬇️

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