Add top level metrics document to summarize monitoring endpoints

[nitisht]

Description

Minio server supports healthcheck and prometheus related
unauthenticated endpoints. This document summarizes this
information in a single place and add links for more detailed
documentation if needed.

Motivation and Context

Detailed Prometheus document is available in Cookbook by
minio/docs#215.

See #5914

How Has This Been Tested?

NA

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 code follows the code style of this project.
• 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 #5923 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5923      +/-   ##
==========================================
+ Coverage   59.57%   59.61%   +0.03%     
==========================================
  Files         214      214              
  Lines       30786    30786              
==========================================
+ Hits        18342    18354      +12     
+ Misses      10899    10886      -13     
- Partials     1545     1546       +1

Impacted Files	Coverage Δ
cmd/update-notifier.go	81.66% <0%> (+19.99%)	⬆️

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[deekoder]

we have to approve cookbook pr first. Change the url and then delete the doc from here