docs: Add v0.7.6 release notes

[matthewfeickert]

Description

Add release notes for pyhf v0.7.6.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add release notes for pyhf v0.7.6.

[matthewfeickert]

Relevant RTD build: https://pyhf--2430.org.readthedocs.build/en/2430/release-notes.html#release-v0-7-6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (release/v0.7.x@1edf68e). Click here to learn what that means.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2430   +/-   ##
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  Coverage                  ?   96.23%           
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  Files                     ?       69           
  Lines                     ?     4487           
  Branches                  ?      611           
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  Hits                      ?     4318           
  Misses                    ?      129           
  Partials                  ?       40           

Flag	Coverage Δ
unittests-3.7	96.23% <0.00%> (?)

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