Added resource exporter

[ekansa]

brief description of changes

Added description of the Resource Exporter group and how it use this group to control search result resource instance data exports.

issues addressed

Version 7.4.0 release notes.

further comments

Temporarily demonstrated here: https://arches.readthedocs.io/en/export_perms_7_4/administering/managing-permissions/#managing-users-and-groups-in-django-admin

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This box must be checked

• [x] the PR branch was originally made from the base branch

This box should be checked

• [x] after these changes the docs build locally without error

This box should only be checked you intend to follow through on it (we can do it on our end too)

• [ ] I will cherry-pick all commits in this PR into other branches that should have them after this PR is merged

[ekansa]

Will self-merge because this is relatively simple update.