Some more README restructuration

This time I think I am clear on what the building & running process is
and what the purpose of each command is, and I tried to make it clear again.

This may be deemed too verbose:
- Maybe I went into too many basic details, I'll be happy to discuss
  what the right level of detail is;
- But I also think there are too many ways to do the same thing
  documented in the README. It might be worth settling on one recommended way,
  and documenting the rest in an appendix or something like that.
4 jobs for 365-documentation-improve-readme in 56 minutes and 45 seconds (queued for 4 minutes and 29 seconds)
latest
Status Job ID Name Coverage
  Cabal
passed #13926
cabal

00:18:36

 
  Stack
passed #13927
stack

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  Bench
manual #13928
allowed to fail manual
bench
 
  Test
passed #13929
test

00:30:07