Remove outdated or duplicated docs for legacy and new workflow system

[sphuber]

Fixes #1717

The documentation about the new workflow system has been completely
rewritten in a single concise piece in concepts/workflows.rst. The
old system had some duplication, which was removed. The leading docs
for the legacy system are in old_workflows/index.rst

[codecov-io]

Codecov Report

Merging #1718 into develop will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1718      +/-   ##
===========================================
- Coverage    57.18%   57.15%   -0.03%     
===========================================
  Files          275      275              
  Lines        33931    33931              
===========================================
- Hits         19402    19393       -9     
- Misses       14529    14538       +9

Impacted Files	Coverage Δ
aiida/backends/djsite/globalsettings.py	81.57% <0%> (-5.27%)	⬇️
aiida/backends/djsite/db/models.py	75.09% <0%> (-0.89%)	⬇️

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