Release note updates

[albeanth]

Description

This PR provides a (very) overdue update to the release notes for some of the PRs I have closed over the past few months.

PRs for "What's new in ARMI":

• Assembly Level Axial Expansion Prototype #606 and implicitly includes
  • Axial expansion update #653
  • Update/upgrade for component axial linkage #670
  • Axial expansion process load bug fix #761
  • Again move axial exp process load #776
  • Thermal expansion update #787
• Update to armi.tests.detailedAxialExpansion #693

PRs for "Bug fixes":

• Uniform mesh deepcopy fix #645
• update helix docstrings + bug fix #747

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works. (N/A)

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.