Add simple examples for cbindlist+mergelist to NEWS

[MichaelChirico]

Would be nice basic examples in news file, so readers can easily jump in to check new functions.

Originally posted by @jangorecki in #4370 (comment)

Let's try and add these before release.

[venom1204]

Hi @MichaelChirico,
I’m planning to add simple examples under the existing cbindlist() and mergelist() entries in NEWS.md. For cbindlist(), I’ll include a basic example showing column-wise concatenation of a list of data.tables. For mergelist(), I’ll include examples demonstrating recursive joins using how = "inner" and how = "full" on a shared id column. Let me know if this sounds good — I’ll proceed to open a PR accordingly.

[MichaelChirico]

Yes. Please focus on keeping the example as concise as possible -- we just want to give the user a quick idea of how the functions work as a teaser, and they can refer to the full help for more details.

[jangorecki]

Full join is rather unusual so I would go with something more common