Add @api stable annotations tilegrid functions and options

This PR adds @api stable annotations to tilegrid-related functions and options.

Things to note:

- `ol.tilegrid.TileGrid` is currenly no used in the examples, but it is needed if you want to use a tiled WMS and display images at arbitrary resolutions. (The `olx.source.TileWMSOptions#tileGrid` option is marked as "stable".)

- I've left `minZoom`, `maxZoom`, and the corresponding `getMinZoom` and `getMaxZoom` functions as "experimental". I think we need to make sure those are consistently used in the various tilegrid implementations. They are not used in the examples.

- `ol.tilegrid.WMTS` is needed when working with WMTS. And it is indeed used in the WMTS examples.

- `ol.tilegrid.XYZ` and `ol.tilegrid.Zoomify` are currently not used in the examples. I haven't seen any reason for making them "stable" for v3.0.0.

Please review.
> see #2314. I don't think this should be marked stable until that bug is fixed

I'm not sure I agree, as we're talking about API stability here.