Spaces docs for serverless

[florent-leborgne]

This PR adds a page about Spaces to the serverless docs as spaces will be included for GA.

This basically adapts the stateful version of the Kibana guide's Spaces page into the serverless docs.

I'll leave this as a draft until I confirm that the following unconfirmed differences with stateful are intended:

• Didn't find any option to copy an object to a different space (removed from this doc in the meantime), leaving the import/export of saved object the only way available to do so.
• Didn't find option to set role privileges on the space, while custom roles should also become available in serverless (so it's not documented here for now)
• Required privileges. With the new serverless user model, the role needed is different. I'll need to confirm whether there are specific roles per solution that allow to do this or if it's just the generic admin role. Also to check if we want to add the specific privilege to use for when custom roles become available.

Note - The following differences were confirmed:

• Feature visibility isn't available on serverless
• New solution view setting isn't available either (which is logical :D)

Still todo: add a more precise link to the serverless Spaces API that isn't published yet. For now I changed the link to be very generic.

Closes: https://github.com/elastic/platform-docs-team/issues/471

[florent-leborgne]

cc @lcawl about the link to the Spaces API for serverless, whenever it gets published we can update the link to it in this PR if it's not yet merged.

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