UG: Enhancend documentation related to variables.
UG: Enhancend documentation related to variables.

- Document new nested item access syntax and recommend accessing
  items using `${var}[item]` syntax, not `@{var}[item]` or
  `&{var}[item]`, in general. Fixes #2601.

- Document new slice syntax with variables containing lists.
  Fixes #2973.

- Some general enhancements.

robotframework/robotframework,HelioGuilherme66/robotframework,robotframework/robotframework,robotframework/robotframework,HelioGuilherme66/robotframework,HelioGuilherme66/robotframework