Document filter_throttle_size plugin

[mrueg]

https://github.com/fluent/fluent-bit/tree/master/plugins/filter_throttle_size doesn't have a page under https://docs.fluentbit.io/manual/pipeline/filters that shows how it's configured and what it is useful for.

It would be nice to have a doc for it as well.

[ganga1980]

looks like this useful filter throttle plugin to use. appreciate the folks who has knowledge on this to add the docs.