Document how to access and manipulate the event payload and metadata

[mfelsche]

Our language section is a bit shy of good conceptual info how to access and manipulate the event payload and metadata in a select statement or a tremor script.

Good places would be:

• https://github.com/tremor-rs/tremor-www/blob/main/docs/language/scripts.md
• https://www.tremor.rs/docs/edge/language/expressions#paths This section could be extended with some info and examples on all the special paths and their usage and contexts
• https://github.com/tremor-rs/tremor-www/blob/main/docs/language/pipelines.md#select-queries

Happy to provide more detailed info for anyone picking this up.