docs: Document HTTPProxy status conditions

[AkashKumar7902]

Fixes #3144

What this PR does

• documents the Contour-managed Valid condition as the detailed HTTPProxy processing result
• explains observedGeneration, errors, warnings, and the derived currentStatus and description fields
• replaces obsolete status examples with current valid and unresolved-Service condition output

Why

The fundamentals page only showed the summary status fields and had not been updated for condition-based status reporting. This left users without documentation for the structured diagnostics Contour currently writes.

User impact

Users can identify the HTTPProxy generation that Contour evaluated and find fatal errors or non-fatal warnings in the correct condition fields.

Release note

Requested label: release-note/docs. The corresponding docs changelog is included.

Testing

• git diff --check
• make lint-codespell
• go test ./apis/projectcontour/v1 ./internal/status ./internal/dag
• hugo --gc --minify with the Netlify-pinned Hugo 0.152.0

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