Improve CLI documentation

[jwodder]

This PR expands the docs for the individual subcommands, adds a page documenting the root dandi command, and adds a single reference page listing the resource identifier patterns for other pages to link to.

[codecov]

Codecov Report

Merging #851 (26c8afe) into master (7ee9da3) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #851      +/-   ##
==========================================
- Coverage   86.83%   86.70%   -0.13%     
==========================================
  Files          60       60              
  Lines        6237     6237              
==========================================
- Hits         5416     5408       -8     
- Misses        821      829       +8     

Flag	Coverage Δ
unittests	86.70% <ø> (-0.13%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/cli/cmd_organize.py	76.87% <ø> (ø)
dandi/cli/cmd_upload.py	55.88% <ø> (ø)
dandi/dandiarchive.py	85.99% <ø> (-2.42%)	⬇️
dandi/download.py	82.44% <0.00%> (-0.54%)	⬇️
dandi/dandiapi.py	89.69% <0.00%> (-0.18%)	⬇️

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