Document space awareness in Fleet

[kilfoyle]

We'll need documentation for the space awareness feature in Fleet.

References:

• meta issue: https://github.com/elastic/ingest-dev/issues/1664
• UI updates: https://github.com/elastic/ingest-dev/issues/2903
• Design doc (see esp. p.17 for UI behaviors)

The feature will be tech preview for 8.14.

Nicolas has offered to give us a feature demo.

[kilfoyle]

Closing as a duplicate of #1142