vcl-built-in-code.rst needs practical examples and clearer structure explanation

[SashankBhamidi]

doc/sphinx/users-guide/vcl-built-in-code.rst explains the concatenation model but doesn't help users decide when to override vcl_builtin_* vs when to use vcl_req_* hooks.

This came up in #4430 (comment) - the file has one example of using vcl_req_cookie (line 46) but no explanation of when you'd use that pattern vs overriding vcl_builtin_recv.

Gaps:

No guidance on choosing patterns
The docs show vcl_req_cookie can override cookie handling but don't explain when you'd override vcl_builtin_recv instead. Users have to read builtin.vcl source to figure this out.

No mention of recursion check
Nowhere does it say that calling vcl_builtin_* from your own vcl_builtin_* won't compile. Users hit this error and don't know why.

Only one practical example
The pipe mode example (lines 189-214) is the only recent practical example. The vcl_req_cookie one (line 46) is there but not explained as a pattern. Need examples showing:

• When to use vcl_req_* vs overriding vcl_builtin_*
• What the recursion error looks like

The vcl_refresh_* section (lines 77-172) is detailed but dense. Most users looking for basic guidance won't read that far.

Ref: #4372

[SashankBhamidi]

I can work on this. Have a draft of what to add but want feedback on approach first.

[nigoroll]

@SashankBhamidi we'll merge #4430 first, then please feel free to suggest other improvements. We people from the core team probably have no good feeling for which questions arise during the learning curve, so we welcome your suggestions for improvements.

[SashankBhamidi]

I have a draft that fixes these gaps.

Added a new section after the built-in subroutines split part that explains when you'd override vcl_builtin_recv versus using the vcl_req_* hooks. The recursion check issue gets an explicit example showing what breaks and why, plus the right way to do it.

The section includes a decision guide so users don't have to guess which pattern to use, and a complete working example that shows all three levels together: preprocessing in vcl_recv, reordering hooks in vcl_builtin_recv, and fine-tuning with vcl_req_cookie.

About 130 lines total, builds cleanly. I can open a PR if this direction makes sense.