docs: add HTTP status codes and import snippet to error-handling page

[yashisrani]

Pull Request Template

fix: #2351

Description:

• Adds a table column that shows the exact HTTP status code returned by each pre-defined error type.
• Added a dedicated “HTTP Status Code” column to the pre-defined HTTP-error table so developers can see at a glance which status GoFr will return (400, 404, 409, 408, 500).
• Placed a concise import snippet at the top of the page showing the exact packages users need (gofr.dev/pkg/gofr/http and gofr.dev/pkg/gofr/datasource).
• No existing prose or code samples were removed—change is purely additive.

Breaking Changes (if applicable):

• None – documentation only.

Additional Information:

• External deps: None beyond the existing GoFr modules.

Checklist:

• I have formatted my code using goimport and golangci-lint.
• All new code is covered by unit tests.
• This PR does not decrease the overall code coverage.
• I have reviewed the code comments and documentation for clarity.

Thank you for your contribution!

[coolwednesday]

@yashisrani, Since this PR is not linked to an open issue. Unfortunately, I would have to close it. Thank you for you contribution, do check out some other issue and only create PR after that issue has been assigned to you.