DOC-2761

This adds a new geo reference page (based mainly on the information here), and also adds missing info about GEOSHAPE to the indexing and schema type info.

A few things to note:

• This breaks the indexing info out of the main indexing page into a geo-specific page. The main indexing page is huge, so I think it should be split into separate pages like this, but doing this for all existing schema types is a job for a separate PR.
• The new geo indexing page will have code samples but I figured it was best to get the text reviewed rather than wait for them all to be approved and merged.
• The JIRA ticket requests both current and planned features, but AFAIK, we don't normally document features until they are available in the product, so I've left these out.

All feedback about content, approach, missing bits, etc, is welcome :-)