* Add a note about index migrations to the kibana setup docs
* Tewak the migrations asciidocs for clarity
* docs: refine saved object migration details
Breaking down the migration process into sections helps people find
and link to relevant information more easily.
The focus is on ongoing maintenance of Kibana, whereas the initial new
experience in 6.5.0 is treated as a note of clarification.
Error handling should be expanded in the future to include details about
specific known error cases.
These changes include all three known upgrade scenarios:
1. New installation (K3 -> K5)
2. Standard upgrade (K4.2 -> K5)
3. Standard upgrade with reindex (K4.1 -> K5)
