AbstractBlockArray documentation enhancement

[kunzaatko]

Recently I wanted to implement the AbstractBlockArray interface for a custom array. I took a look at the documentation, but it does not provide sufficient instructions for the user to provide the correct method implementations.

This PR should serve as an attempt to improve this. The template documentation should be the AbstractArray interface in the standard library. This consists of two features:

1. A table that states the methods that are necessary to implement with default implementations and descriptions.
2. An example implementation of some toy array type which emphasizes the important things to keep in mind and explains the types that are at the input of the methods and that should be at the output of the methods.

The current commits only fix some obvious issues with the documentation and the docstrings which bring it closer to the standard library documentation and docstrings of the AbstractArray types. These should be non-controversial. It is only a draft, since the documentation of the interface needs a greater overhaul and this should provide a place to
discuss the options for this. Should I proceed in giving it a try or do you have other intentions with the documentation @jishnub? I want to assure myself before doing work that might be useless...

[jishnub]

This seems pretty useful to me. Please proceed with a draft, and we may iterate on the details.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.62%. Comparing base (a80fcf7) to head (79bf530).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #456       +/-   ##
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+ Coverage    0.00%   93.62%   +93.62%     
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  Files          18       19        +1     
  Lines        1642     1664       +22     
===========================================
+ Hits            0     1558     +1558     
+ Misses       1642      106     -1536     

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