Improve cli commands

[MJGaughran]

Improve the behaviour of CLI commands and flesh out help info.

Also adds a couple improvements to the README.

[codecov]

Codecov Report

Attention: Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 44.38%. Comparing base (e78dc0f) to head (e1edeba).

Files with missing lines	Patch %	Lines
src/deploy_tools/compare.py	66.66%	3 Missing ⚠️
src/deploy_tools/validate.py	0.00%	3 Missing ⚠️
src/deploy_tools/__main__.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   43.77%   44.38%   +0.61%     
==========================================
  Files          23       23              
  Lines         795      811      +16     
==========================================
+ Hits          348      360      +12     
- Misses        447      451       +4     

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