docs: add man and markdown documentation

[TobiaszCudnik]

Because

• increase the visibility of the built-in --help documentation

This commit

• add man and markdown docs, along with a make doc generator

[linear[bot]]

INS-2115 CLI: documentation

• [X] --help args
• [ ] readme
  • [ ] PR https://github.com/instill-ai/model-backend/pull/421/files
  • [ ] usage examples
  • [ ] dev build steps
• [ ] man page

Later

• [ ] integrate with instill.tech/docs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a9d3b4b) 51.09% compared to head (ac231af) 51.09%. Report is 28 commits behind head on main.

:exclamation: Current head ac231af differs from pull request most recent head 329ea91. Consider uploading reports for the commit 329ea91 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #185   +/-   ##
=======================================
  Coverage   51.09%   51.09%           
=======================================
  Files          54       54           
  Lines        4196     4196           
=======================================
  Hits         2144     2144           
  Misses       1833     1833           
  Partials      219      219           

Flag	Coverage Δ
unittests	51.09% <ø> (ø)

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