[docs] added docs for input, schedule and condition
[docs] added docs for input, schedule and condition

Also:

- removed `throttle` file (throttling is not covered in `alert-anatomy` page)
- fixed ctx model parameters in templates/scripts to use the `ctx` prefix.
- added a dediated section in alert-anatomy about execution context as template/script model
- wrote first page of `transform` section

Original commit: elastic/x-pack-elasticsearch@471ca7f0d9b2f8e142f8d3cd6c616bd18c8dbaed

uschindler/elasticsearch,nknize/elasticsearch,strapdata/elassandra,robin13/elasticsearch,uschindler/elasticsearch,robin13/elasticsearch,GlenRSmith/elasticsearch,strapdata/elassandra,vroyer/elassandra,robin13/elasticsearch,HonzaKral/elasticsearch,gfyoung/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,robin13/elasticsearch,gingerwizard/elasticsearch,vroyer/elassandra,HonzaKral/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,nknize/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,scorpionvicky/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,strapdata/elassandra,coding0011/elasticsearch,strapdata/elassandra,robin13/elasticsearch,uschindler/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,vroyer/elassandra,gfyoung/elasticsearch,coding0011/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,scorpionvicky/elasticsearch,strapdata/elassandra,uschindler/elasticsearch