Update "Accessing the Overture Catalog" to lead with STAC and correct GERS artifact descriptions

[danabauer]

Type

Task

Scope

Multi-theme or Platform

Skillset

None

Description

Things to fix in docs/getting-data/cloud-sources.mdx

• Rewrite the intro to frame the page as the Overture catalog, everything Overture publishes with each release, and present STAC as its index.
• Move the STAC section to the top and describe what the catalog contains: the latest field and GERS registry manifest at the root, a PMTiles link per theme, and per-type collections with spatial extent, feature count, and column names.
• Note that bridge files and the changelog aren't in the STAC catalog yet.
• Update the GERS artifacts descriptions
• Add a warning that the PMTiles are for data inspection only and shouldn't be used in production.