Add Marbella beach price dataset to economic data

[shiftdylson1]

Adds a small, single-city open dataset to economic-data.md (Other economic data and indicators): a seasonal census of sunbed, beach club and chiringuito prices in Marbella, Spain.

• Source: each venue's own published rates, with per-venue provenance notes on what was read and when. Published prices, not transactions or estimates.
• CC BY 4.0, with a Zenodo DOI (10.5281/zenodo.22094846) and CSV/JSON downloads.
• Repo: https://github.com/shiftdylson1/marbella-price-data

Flagging the scale honestly: this is a single-city consumer-price dataset sitting next to global indicators, so it is smaller in scope than most entries on that page. I placed it there because it is the closest topical fit and there is no tourism or local-prices page; happy to move it, put it elsewhere, or close this if you would rather that page stay at national/global scale. The recently merged Phuket condo entry on property-prices.md seemed like a reasonable precedent for a local, independently published price dataset.