TMS and Vector Tiles

[m-mohr]

Clarify how XYZ relates to TMS and vector tiles.

We could for example:

• Allow {-y} in XYZ and clarify that the xyz rel type supports also TMS
• Add a new vector-tile rel type that is pretty much aligned with XYZ, but allows specifically vector tiles. We'd then need to clarify that XYZ is raster.
• Both points above may also be a first entry point for OGC API - Tiles (Add OGC API - Tiles #12).