v0.28: `_geo` accepts string values

[guimachiavelli]

Starting with v0.28, _geo fields (both lat and lng) should accept numbers in both numeric and string formats:

"_geo": {
  "lat": 0.0, // floating point number, valid
  "lng": "0.0" // a string that can be adequately converted into a float, also valid
}

We need to update the Geosearch guide so this is stated clearly.

References

Spec: meilisearch/specifications#83
Issue on core: meilisearch/meilisearch#2354
SME: @irevoire