Add 'mc config host add' command in multi-user doc

[harshavardhana]

Description

Add 'mc config host add' command in multi-user doc

Motivation and Context

Add 'mc config host add' command in multi-user doc

Regression

No

How Has This Been Tested?

Following the multi-user doc https://docs.minio.io/docs/minio-multi-user-quickstart-guide.html

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added unit tests to cover my changes.
• I have added/updated functional tests in mint. (If yes, add mint PR # here: )
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #6777 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6777      +/-   ##
==========================================
- Coverage   52.09%   52.08%   -0.01%     
==========================================
  Files         270      270              
  Lines       42322    42322              
==========================================
- Hits        22046    22044       -2     
- Misses      18330    18332       +2     
  Partials     1946     1946

Impacted Files	Coverage Δ
cmd/posix-list-dir_windows.go	58.82% <0%> (-5.89%)	⬇️
cmd/posix.go	70.28% <0%> (-0.25%)	⬇️
pkg/certs/certs.go	58.76% <0%> (+4.12%)	⬆️

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