[Spaces] document how to move objects between spaces

[AlonaNadler]

With Spaces being release, users will want to move objects between spaces. The way to do it is by exporting objects and importing into the new Space. Currently, the import/export UI doesn't export with dependencies, so it will be challenging to select specific dashboard from a huge list of objects and then track the dependencies.
Until this will address #16831 users can use the import/export APIs that export with the dependencies. It will be extremely helpful if we have a doc that explains how to use it in the context of Spaces
There are 2 use cases:

• Within a single Kibana instance moving specific dashboard to a new space with the dependencies
• In case users have multiple instances of Kibana for a single ES cluster and they want to consolidate these instances now that Spaces are available. A doc that explains how to export from a Kibana instance and import to space with all the dependencies

cc: @kobelb @legrego @jinmu03 @gchaps

[legrego]

Resolved via #23205