[FEATURE] Concurrent page extraction

[gunnsth]

Is your feature request related to a problem? Please describe.
Currently extraction only supports processing pages one by one. It might be more efficient to use multiple go-routines to handle page-by-page.

Describe the solution you'd like
Explore what the easiest way to support concurrency in extractor package is.

Describe alternatives you've considered
Alternative and currently the best way for concurrency is on a document basis. I.e. one go-routine handling a single document.

Additional context
Client's comment

We often deal with documents that are 900+ pages and serially processing these with Unidoc was. Taking a long time and this a lot of money in AWS expenses.

[anovik]

@gunnsth This is done, here is the example how to do that: https://github.com/unidoc/unipdf-examples/blob/master/concurrent-processing/concurrent_extraction_page_level.go