Document BatchedCommand and BatchedAnnex

[christian-monch]

This PR adds design documentation for BatchedCommand and BatchedAnnex, as well as a very basic test for BatchedCommand. This PR is a fallout of my work on implementing BatchedCommand and BatchedAnnex with the new generator-based runner. Although this PR might be outdated in the next or a later release, I think it provides some value.

• Add documentation for BatchedCommand and BatchedAnnex
• Add a basic test

[mih]

God, yes! Thank you!

[codecov]

Codecov Report

Merging #6203 (0b69e78) into master (02f6914) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6203   +/-   ##
=======================================
  Coverage   89.71%   89.71%           
=======================================
  Files         319      319           
  Lines       41717    41727   +10     
=======================================
+ Hits        37426    37436   +10     
  Misses       4291     4291           

Impacted Files	Coverage Δ
datalad/tests/test_cmd.py	100.00% <100.00%> (ø)

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[mih]

Maybe a little side-note that I got initially confused by the terminology. "command" is not the thing that is "batched" (as in batchedCommand). When the term "command" is used, it is one of the stdin items that are passed to the running command-batch-process. But I am not sure how to do this better. It is not an argument either.