DOC: Orthogonalize docs with handbook content

[mih]

Concept: All docs remaining in here are technical and developer-focused. The handbook has acquired all previously existing content and use case descriptions, and is now a complete and more comprehensive replacement for any user/usage-focused information.

This commit strips away anything that is duplicate, while keeping executable code that can still serve a purpose in the test battery.

In a future pass through the docs, and after the structure and content of the handbook has matured a bit more, content here can be further refined and more (and more useful) cross-references can be added.

Potential TODOs (please give feedback):

• the content of chapter basic principles is essentially contained in the handbook already, hence could be removed too (as it is not developer-focused). The only two things missing are:
  • info on ID types (this should become a dedicated chapter on the general metadata/identification approach that should be approached in the metalad-context (note to self @mih)
  • "API principles", which has only few principles, and is mostly an assorted collection of information otherwise -- which might be useful to place on a cheat sheet datalad cheat sheet datalad-handbook/book#139 (ping @adswa for awareness)

[codecov]

Codecov Report

Merging #3678 into master will decrease coverage by 5.39%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #3678     +/-   ##
=========================================
- Coverage   82.48%   77.08%   -5.4%     
=========================================
  Files         274      274             
  Lines       35805    35964    +159     
=========================================
- Hits        29532    27722   -1810     
- Misses       6273     8242   +1969

Impacted Files	Coverage Δ
datalad/support/stats.py	32% <0%> (-65.34%)	⬇️
datalad/core/local/diff.py	34.61% <0%> (-61.49%)	⬇️
datalad/interface/unlock.py	43.66% <0%> (-54.91%)	⬇️
datalad/support/keyring_.py	35.55% <0%> (-48.89%)	⬇️
datalad/distribution/create_sibling_github.py	36.06% <0%> (-47.55%)	⬇️
datalad/interface/rerun.py	14.9% <0%> (-47.28%)	⬇️
datalad/ui/base.py	50% <0%> (-45%)	⬇️
datalad/ui/dialog.py	49.41% <0%> (-40.53%)	⬇️
datalad/distribution/siblings.py	14.46% <0%> (-39.7%)	⬇️
datalad/interface/clean.py	49.09% <0%> (-38.19%)	⬇️
... and 109 more

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[mih]

OK, let's go with this state for now.