Add exhaustive list of flow variable expressions

[wrussell1999]

Description

A few things need updating:

• Expressions are the template, not the variable itself. However variables exist as something else. How can we clarify this?
• There isn't a full list of all the expressions you can write that use all the flow variables (e.g. flow.id) in one place with links to subsection if explained in detailed in a different page/secton.
• This flows page needs more info on flow variable expressions.

From app: https://github.com/kestra-io/kestra/blob/develop/ui/src/assets/documentations/basic.md

[wrussell1999]

Number 3: 93b10e4