Automatically generate examples documentation adv (#19294)

[cj-zhukov]

Which issue does this PR close?

• Closes #Automatically generate examples documentation and add CI sync check #19294.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[cj-zhukov]

High-Level Overview

In previous PRs, a basic script was introduced that only parsed example group names #19371 and #19491

This PR implements a more comprehensive solution for keeping the examples README up-to-date:

1. Introduces a new examples.toml file, which contains metadata for examples, including:

• subcommand — the command used with cargo run --example [group] -- [subcommand]
• file — the actual Rust source file
• desc — a short description of the example

2. Updates the generate_examples_docs.sh script to:

• Parse examples.toml
• Check that all examples in the filesystem exist and are listed in the TOML
• Generate a new README file as README-NEW.md

3. Integrates CI validation:

• README-NEW.md is formatted with DataFusion’s Prettier
• Compared against the committed README.md
• If there are differences, the CI check fails and prints a clear diff, along with instructions for updating the README after verifying examples.toml

This ensures that the README for examples stays in sync with the actual examples and encourages maintainers to keep examples.toml accurate.

[cj-zhukov]

@Jefffrey since you helped with previous PRs related to example documentation, it would be great if you could take a look at this one as well. Your feedback on the CI check approach or any improvements would be much appreciated.