Add objects.inv to documentation

[evetion]

using the latest Documenter. So we can link to GeoInterface documentation from other websites (e.g. evetion/GeoDataFrames.jl#135).

Per https://juliadocs.org/DocumenterInterLinks.jl/stable/write_inventory/#Inventory-Generation:

For Julia projects that build their documentation with Documenter ≥ v1.3, an objects.inv inventory file in the same format is automatically created in the docs/build/ folder.

[asinghvi17]

This should be autogenerated, if it's not then we need to either pass some kwarg to Documenter.HTML or fix the Documenter version