docs: add FUNC_USERNAME/PASSWORD env vars documentation

[Kunal1522]

Fixes knative/func#3299

Proposed Changes

• Added documentation for FUNC_USERNAME and FUNC_PASSWORD environment variables in docs/snippets/proc-building-function.md.
• Added documentation for FUNC_USERNAME and FUNC_PASSWORD environment variables in docs/snippets/proc-deploying-function.md.
• Clarified that these environment variables now work with all builders (host, s2i, and pack) for private registry authentication, following the refactoring in knative/func#3298.
• Since these are snippets, the updates will automatically appear in the "Building functions", "Deploying functions", and "Getting started" documentation pages.

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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Kunal1522 / name: Kunal Kashyap (bff0eea)

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[knative-prow]

Welcome @Kunal1522! It looks like this is your first PR to knative/docs 🎉

[Cali0707]

/cc @gauron99 @lkingland

[gauron99]

Thanks for the ping. I commented on the mentioned issue here