feat: Gen cli docs

[markphelps]

re: FLI-608

• adds hidden subcommand to enable us to generate the CLI docs for copy/pasting into our actual documentation

Usage

$ flipt docs
$ tree tmp/docs/
tmp/docs/
├── flipt.md
├── flipt_config.md
├── flipt_config_edit.md
├── flipt_config_init.md
├── flipt_export.md
├── flipt_import.md
├── flipt_migrate.md
└── flipt_validate.md

1 directory, 8 files

[codecov]

Codecov Report

Merging #2173 (d543fbe) into main (17ba0ee) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2173   +/-   ##
=======================================
  Coverage   70.54%   70.54%           
=======================================
  Files          76       76           
  Lines        7237     7237           
=======================================
  Hits         5105     5105           
  Misses       1833     1833           
  Partials      299      299           

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