Chapter 5 update

[OisinJohnston2005]

Fixes issues with chapter 5 of the kpt book, some urls were outdated (stuff like moving from master branch to main) and the get-started repo's code is outdated in the current version

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[Copilot]

Pull request overview

Updates Chapter 5 of the kpt book to align the “developing functions” walkthrough with current upstream repositories/branches and the latest get-started scaffolding.

Changes:

• Switches the kpt pkg get example from @master to @main.
• Updates the embedded Go main.go example to include fn.WithDocs(...) and go:embed assets.
• Adjusts testdata/example paths and a shell pipeline command used in the walkthrough.

Comments suppressed due to low confidence (1)

documentation/content/en/book/05-developing-functions/_index.md:248

• The testdata instructions are inconsistent: the comment points to testdata/noop-passthrough/resources.yaml, but the edit command uses testdata/test1/resources.yaml, and the subsequent kpt fn source sources the whole testdata directory. This makes it unclear which resources are being tested.

# Edit the `testdata/noop-passthrough/resources.yaml` with your KRM resources. 
# resources.yaml already has a `Deployment` and `Service` as test data. 
vim testdata/test1/resources.yaml

# Convert the KRM resources and FunctionConfig resource to `ResourceList`, and 

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[liamfallon]

Once the copilot comments are resolved we can go for a human review.

[efiacor]

rebase required