Change digital object documentation wording from "Master"

Hi folks, I'd like to make a pull request (after discussions with team manager) for changing documentation wording to reflect updated terminology with regard for our work towards ensuring anti-racist and inclusive language is used in our documentation so that it can match the words we use when talking about our software. I can go through an make a pull requests for all instances where "Master" is used but I first wanted to give you folks the chance to view this issue and discuss if needed.

This happens throughout the documentation (on v2.8, 2.7, and earlier versions) but some instances can be found here:
https://www.accesstomemory.org/en/docs/2.8/user-manual/add-edit-content/rights/#make-rights-actionable-on-digital-objects
https://www.accesstomemory.org/en/docs/2.8/user-manual/import-export/upload-digital-object/
https://www.accesstomemory.org/en/docs/2.8/user-manual/administer/settings/#digital-object-derivatives
(I am happy to provide links to all instances of the terminology if you wish!).

As well we define "Master digital object" in the glossary:
https://www.accesstomemory.org/en/docs/2.8/user-manual/glossary/glossary/#term-master-digital-object

We are using the wording "original" but happy to make the pull request be a different term. In general - a good overview of terms can be found here: https://www.it.northwestern.edu/about/it-projects/dei/glossary.html

Github has shifted their terminology from master to main.

Start with the three v2.8 user-manual pages linked in the issue and the glossary entry for “Master digital object”; review the proposed “original” wording and the linked terminology guide. Search the v2.8, v2.7, and earlier documentation for all instances, then confirm that each occurrence uses the agreed replacement consistently.

Written by the indexing model from the issue text.