Improve Cost Saving Calculator as a conversion and SEO page

[liunyl]

Epic: #47

Background

/costsaving is a strong conversion page and should support Redis cost reduction queries while clearly documenting model assumptions.

Tasks

• Update title and meta description for Redis/ElastiCache cost reduction intent.
• Add explanatory copy describing the cost model and assumptions.
• Add preset links or shareable states if feasible.
• Add FAQ about Redis cost drivers, DRAM, replicas, NVMe, and workload fit.
• Link from results to product page, benchmark, and contact/signup.
• Add structured data where appropriate.

Acceptance criteria

• Calculator page is understandable without interacting first.
• Cost assumptions are visible and caveated.
• Page has clear next steps after calculation.