Add docs for subcommands

[tiangolo]

✨ Docs for subcommands, updated tests, and others.

Part of several local changes still to be pushed.

[codecov-io]

Codecov Report

Merging #28 into master will increase coverage by 2.63%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   97.32%   99.95%   +2.63%     
==========================================
  Files          99      135      +36     
  Lines        1983     2388     +405     
==========================================
+ Hits         1930     2387     +457     
+ Misses         53        1      -52

Impacted Files	Coverage Δ
...est_subcommands/test_name_help/test_tutorial008.py	100% <100%> (ø)
docs/src/subcommands/name_help/tutorial004.py	100% <100%> (ø)
...test_tutorial/test_subcommands/test_tutorial002.py	100% <100%> (ø)
...ommands/test_callback_override/test_tutorial004.py	100% <100%> (ø)
...est_subcommands/test_name_help/test_tutorial006.py	100% <100%> (ø)
...test_tutorial/test_subcommands/test_tutorial001.py	100% <100%> (ø)
docs/src/subcommands/tutorial003/users.py	100% <100%> (ø)
...ommands/test_callback_override/test_tutorial003.py	100% <100%> (ø)
...ommands/test_callback_override/test_tutorial001.py	100% <100%> (ø)
docs/src/subcommands/tutorial001/users.py	100% <100%> (ø)
... and 64 more

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