doc: Enhance <let/> documentation

I appears that doc for `` element is a bit too basic, doesn't introduce many advance scenarios like combination with list filtering with value converter, binding behavior. This could make it underused. Would be nice if there could be more documentation about those usages of ``.
@EisenbergEffect Please label this `help wanted` + `documentation`

Start by locating the existing documentation for the `` element in this repository and review how list filtering, value converters, and binding behavior are currently documented. Done means the documentation explains these advanced usages with useful examples and makes the element easier to understand and use.

Written by the indexing model from the issue text.