docs: Add v0.7.6 release notes

[matthewfeickert]

Description

• Forward port PR docs: Add v0.7.6 release notes #2430 and docs: Correct PR number in release notes #2432 from release/v0.7.x to main.
• 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

* Forward port PR #2430 and #2432 from release/v0.7.x to main.
* Add release notes for pyhf v0.7.6.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8be3e90) 98.28% compared to head (1b959ea) 98.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2434   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files          69       69           
  Lines        4539     4539           
  Branches      803      803           
=======================================
  Hits         4461     4461           
  Misses         45       45           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.86% <ø> (ø)
doctest	60.71% <ø> (ø)
unittests-3.10	96.29% <ø> (ø)
unittests-3.11	96.29% <ø> (ø)
unittests-3.8	96.32% <ø> (ø)
unittests-3.9	96.34% <ø> (ø)

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