cli reference overview base cmd

[dvdksn]

- What I did

Moves the cli.md file to become the docker.md base command reference.

This file contains a lot of information about how the CLI and its flags work, base flags, configuration file, and environment variables. I think it makes more sense to have it on the base command page than in a separate place.

Needs:

• https://github.com/docker/cli-docs-tool/pull/53

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

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

Project coverage is 61.48%. Comparing base (1dee86b) to head (dc22572).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5010   +/-   ##
=======================================
  Coverage   61.48%   61.48%           
=======================================
  Files         298      298           
  Lines       20814    20814           
=======================================
  Hits        12797    12797           
  Misses       7105     7105           
  Partials      912      912