Add to docs how to resolve target page of outlines entries

[dmlls]

Pull request

This PR adds to the docs how to resolve the target page number of Outlines entries. In #813 it has been raised whether this should be directly exposed by pdfminer.six, but since it's still unclear whether this will happen, at least we can provide a way of doing it in the current state of things.

How Has This Been Tested?

I've tested the code snippets included in the added documentation with 290 PDF documents. Unfortunately, I will have to stick to the classic "I cannot share these documents." But here there is a Colab with a few docs.

The only thing I have not be able to test is SE entries since I haven't been able to find any doc containing them. If anyone can provide such document I'd be happy to test that too.

Checklist

• I have read CONTRIBUTING.md.
• I have added a concise human-readable description of the change to CHANGELOG.md.
• I have tested that this fix is effective or that this feature works.
• I have added docstrings to newly created methods and classes.
• I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.

[pietermarsman]

And thanks again! Looks great!