docs: raster basemaps need an API key and are being retired

[jatorre]

Adds the API key requirement and the raster retirement notice to the README.

Companion PRs: CartoDB/gitbook-documentation#347 (the FAQ) and CartoDB/carto-website (the landing page these links point at).

Why this repo matters most

We are watermarking unauthenticated raster tiles with "API key required". This README is almost certainly the largest single source of the traffic that will hit it: section 1 publishes the raster URL template and a working copy-paste Leaflet snippet, the repo is public, and the basemaps FAQ links here for "predefined basemap styles". Someone who copied that snippet years ago has never been told that terms apply to it.

What changed

Two edits, both text:

1. The terms line at the top now leads with the key requirement and links both the key request page and carto.com/legal/bmap — previously it pointed only at carto.com/basemaps.
2. A callout directly above section 1, not at the top of the README. Someone scrolling to grab the snippet will not read a banner 20 lines above the fold, so the notice sits where the URLs are: keys are free within the fair use limit, and new applications should use the vector styles in section 3 instead.

Also fixed the stale carto.com/location-data-services/basemaps/ link in section 1 to carto.com/basemaps.

Deliberately not in here

The Leaflet snippet is unchanged. How the key travels — query string, header, or subdomain — is still open with @ManuelLR, and documenting a parameter name before that is decided would mean shipping something to be corrected. The callout states the requirement and routes to the landing page for the how.

Worth noting separately: that snippet also omits the {style} path segment (it reads .../{z}/{x}/{y}, with no style), so as written it does not work. Left alone here to keep this diff to the notice, but it deserves its own fix.

Worth a look while we are here

The snippet hardcodes maxZoom: 20, and production has no zoom ceiling. Deep-zoom raster is a meaningful share of the CDN bill, so this line has cost attached to it.

⚠️ Merge order

Blocked on the landing page at carto.com/basemaps/key existing — both links here point at it — and on the terms gaining a free tier, since the live Basemaps Terms currently have none. See the carto-website PR.