Update links in documentation to point at new example locations

[alamb]

Which issue does this PR close?

• part of Reduce the number of distinct binaries to reduce CI / compile time #18142

Rationale for this change

@cj-zhukov has been consolidating the examples, but that also means that the links to the examples are now out of date

What changes are included in this PR?

Update links (I used openai codex, with the prompt below) and reviewed the results

Prompt

We have moved the examples around in datafusion-examples/

  There are some links in doc comments such as in `datafusion/src/lib.rs`:

  [`expr_api`.rs]: https://github.com/apache/datafusion/blob/main/datafusion-examples/
  examples/expr_api.rs

  This example has been moved to ./datafusion-examples/examples/query_planning/
  expr_api.rs

  Please look for all other links to datafusion-examples and update them if necessary to
  point at the new path.

Are these changes tested?

By CI

Are there any user-facing changes?

Fixed docs

[Jefffrey]

Oh I just ran into this when looking up some documentation, nice timing 😄

[alamb]

I don't think this is a controversial change so I am just going to merge it in now. I am happy to make any changes if something comes up afterwards

[alamb]

Thanks @Jefffrey and @cj-zhukov