add docs for analysis.align.AverageStructure

[orbeckst]

Fixes #2943

Changes made in this Pull Request:

• backport PR add docs for analysis.align.AverageStructure #2944 to 1.0.x
• cherry picked from commit 7cae034
• Add missing docs for AverageStructure
• clean up versionchanged for AverageStructure

PR Checklist

• n/a Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2946 into master will increase coverage by 0.69%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2946      +/-   ##
==========================================
+ Coverage   91.45%   92.14%   +0.69%     
==========================================
  Files         168      171       +3     
  Lines       23391    23728     +337     
  Branches     3186     3186              
==========================================
+ Hits        21393    21865     +472     
- Misses       1372     1766     +394     
+ Partials      626       97     -529     

Impacted Files	Coverage Δ
package/MDAnalysis/analysis/align.py	97.85% <ø> (+2.50%)	⬆️
package/MDAnalysis/topology/tpr/obj.py	96.82% <0.00%> (-3.18%)	⬇️
...onality_reduction/DimensionalityReductionMethod.py	97.14% <0.00%> (-2.86%)	⬇️
package/MDAnalysis/analysis/base.py	100.00% <0.00%> (ø)
package/MDAnalysis/lib/_augment.pyx
package/MDAnalysis/lib/c_distances.pyx
package/MDAnalysis/lib/qcprot.pyx
package/MDAnalysis/tests/__init__.py	100.00% <0.00%> (ø)
package/MDAnalysis/due.py	56.09% <0.00%> (ø)
package/MDAnalysis/analysis/legacy/__init__.py	0.00% <0.00%> (ø)
... and 92 more

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