[SC-25388] docs(logs): rewrite Script Field Processor page for CEL

[codeParyul]

Goal

The SCRIPT_FIELD pipeline processor engine moved from Painless to CEL — Common Expression Language (SC-25285, sematext-cloud PR #8149). This rewrites https://sematext.com/docs/logs/script-field-processor/ to match the deployed engine, alongside the editor update in sematext-cloud PR #8210 (SC-25388).

Changes

• Language is CEL (linked to https://cel.dev/), a single expression per script.
• doc['field'] / doc.field access; bracket form for dotted names; missing fields read as null.
• Numeric fields need explicit int() / double() conversion — log bodies are decoded with UseNumber, so numbers reach CEL string-backed; bare doc['size'] * 2 fails at runtime. All numeric examples show the conversion.
• has() presence test, with doc['os.host'] != null for dotted names (has() can't take bracket syntax).
• String helpers, cel.bind(), and map results setting several fields at once (target then ignored).
• No loops/comprehensions (rejected at save); runtime script failure leaves the event unchanged, never drops it.
• Replaced the Painless loop example (inexpressible in CEL) with verified CEL examples — every example on the page was executed against the actual engine environment, including the old page's kb-extraction example, whose substring(17, …) was off by one (produced "42", not "142").

Verification

All claims and examples were validated by running them against the real scriptEnv from go/consumer/cmd/logs/pipelines/proc/script.go (cel-go v0.20.1) with production-shaped (UseNumber-decoded) records.