Changes:
* Updates the write index snippet to use data streams
* Notes data streams aliases don't set an implicit write stream, even if the alias points to one data stream.
This allows indexing documents into a data stream alias.
The ingestion is that forwarded to the write index of the data stream
that is marked as write data stream.
The `is_write_index` parameter can be used to indicate what the write data stream is,
when updating / adding a data steam alias.
Relates to #66163