Fix 6.3 migrate docs (#29640)
Fix 6.3 migrate docs (#29640)

The 6.3 migrate docs are missing some formatting so they render poorly
(all changes land under plugins). This commit fixes these issues
breaking the 6.3 migrate docs into three sections (API, plugins, and
settings).
vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra