feat(landing): New Relic pricing guide - #1568

🟢 Confidence 8/10 · likely safe to merge. Held at 8 because earlier reviews of this pull request found 3 defects.
_{no findings · tests not needed · risk low · 3 defects fixed in earlier rounds}

Adds the /guides/new-relic-pricing landing page, lists it in the guides index and llms.txt, and lets the last guides card fill an xl grid hole. Content-only Astro changes; safe to merge.

• New new-relic-pricing.astro guide page with plan, free-tier and worked-example sections
• Guide listed in coreGuides and in llms.txt.ts
• Last guides card gets xl:col-span-* from coreGuides.length % 3

_{5194b5f · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}