change all api key/rate limit pages to link to documentation/overview

[kkowalsky]

because updating things is hard, we should change the 'api-keys-rate-limits.md' file to link to the documentation overview page to prevent huge changes to a bunch of docs repos

[rmglennon]

• [x] mapzen.js - see https://github.com/mapzen/mapzen.js/pull/373
• [x] cartography, no changes needed
• [ ] Tangram - needs changes to JS tutorial https://mapzen.com/documentation/tangram/walkthrough/ (which should be reviewed anyway)
• [x] Vector tiles - see #291 Update here for API keys: https://mapzen.com/documentation/vector-tiles/migrate-to-vector-tiles-v1/
• [x] Terrain tiles - see #291
• [x] Search - see https://github.com/pelias/pelias-doc/pull/189, pelias/pelias-doc#190, and #289
• [x] All the Valhalla services - see https://github.com/valhalla/valhalla-docs/pull/165
• [x] Metro Extracts - needs to be updated for email accounts and not just GitHub auth https://github.com/mapzen/metro-extracts/pull/308

[rmglennon]

@kkowalsky - these should all be ready for review when you want to look. The remaining item is the old Tangram tutorial, which we can talk about because it needs some other updates.

Thanks!

[kkowalsky]

closing this issue and moving tangram into another issue