Add the merging PDFs together example as a crate function

[DaniD3v]

I think that merging two PDFs together is a very common use case. Would you mind adding that code to the library instead of just adding it as an example? I more or less copy-pasted it for now but that's not exactly a great solution.

I'm aware that this is a low-level library and a PDF merger doesn't exactly fit that category. If you're worried about that (or the unnecessary bloat it would add to the library) could you make it a new crate?

I know that I'm asking for a lot here but there's currently not really a crate that does that well. I even tried the more high level lopdf crates but they didn't work out for me.

[J-F-Liu]

I'm aware that this is a low-level library and a PDF merger doesn't exactly fit that category. If you're worried about that (or the unnecessary bloat it would add to the library) could you make it a new crate?

I don't have this worry, PR is welcome

[williamdes]

@Heinenen How do you view this request?
It would definitely be a great feature to replace existing pdf mergers at work for example.

If some features are not implemented here, will this break while merging?

[Heinenen]

Sounds good.

I think this issue only aims to have the second example in the README "Merge PDF documents" wrapped in some library function.
This would not add any functionality to lopdf, only convenience.

I can't speak to the current quality of the merging.