Add Document get_index_from_location / get_location_from_index

[davidbrochart]

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[davidbrochart]

I think these methods could be useful when some document changes come from a system which is not Location-aware, but only reports changes pointing to raw indexes in the document content, like in collaborative editing.

[willmcgugan]

@darrenburns Could you have a look at this one pls.

[davidbrochart]

Thanks for the review @darrenburns, I made the changes, rebased and added some tests.

[darrenburns]

Thanks @davidbrochart, could you also update the changelog?

[davidbrochart]

Done.

[darrenburns]

Thanks!