docs: add docs and example showing how to get the expression data type

[r3stl355]

Which issue does this PR close?

Closes #7725

Rationale for this change

Make it easier to understand what the get_type returns

What changes are included in this PR?

Are these changes tested?

An extra example in datafusion-examples/examples/expr_api.rs
Docscring in datafusion/expr/src/expr_schema.rs
A paragraph in docs/source/library-user-guide/working-with-exprs.md

Are there any user-facing changes?

Only documentation and an example, no breaking changes

[r3stl355]

Few issues with the docstrings. I can resolve some by using lib names instead of fully qualified (e.g. datafusion_expr instead of datafusion::logical_expr) but arrow_schema::DataType requires an arrow-schema dependency which I don't want to add to Cargo.toml just for this. Should I mark the docstring as ignore?

Ignore this comment, found another way

[alamb]

Thank you @r3stl355 -- this looks great ❤️ . It will be very helpful

I had some small suggestions but I also think we could do them as follow on PRs (or never)

[r3stl355]

I had some small suggestions but I also think we could do them as follow on PRs (or never)

@alamb I'll make these changes later today, this is not merged yet so I don't see a reason for not implementing them

Update: I made all the suggested changes

[alamb]

Thanks again @r3stl355