[DOCS] add azure and gce discovery plugins

Clean EC2 disco doc
Add Azure disco doc
Add Google Compute Engine doc
Fix Zen doc (add `enabled` in `multicast` parameters list) - Fix #5032.
Ignore case when parsing `script_values_sorted|unique` in aggregations.

Close #5009
[DOCS] Removed the experimental label from Tribe, Hot Threads
and Completion Suggester
[DOCS] Updated migration docs for multi_field to point to copy_to
[Docs] value_count supports script per 1.1
Added `cross_fields` mode to multi_match query

`cross_fields` attemps to treat fields with the same analysis
configuration as a single field and uses maximum score promotion or
combination of the scores based depending on the `use_dis_max` setting.
By default scores are combined. `cross_fields` can also search across
fields of hetrogenous types for instance if numbers can be part of
the query it makes sense to search also on numeric fields if an analyzer
is provided in the reqeust.

Relates to #2959