Docs: create metrics documentation

[dreamorosi]

Description of the feature request

Problem statement
N/A.

Summary of the feature
Documentation for the Metrics library:
https://awslabs.github.io/aws-lambda-powertools-python/core/metrics/

Code examples
https://github.com/awslabs/aws-lambda-powertools-python/tree/develop/docs
https://github.com/awslabs/aws-lambda-powertools-python/blob/develop/README.md

Benefits for you and the wider AWS community
Understand how to use the library and speed up their adoption.

Describe alternatives you've considered

N/A

Additional context

N/A

Related issues, RFCs

#25
#152

[dreamorosi]

Working branch with base skeleton of documentation here, you can run the docs locally with npm run docks-local-docker.

[github-actions]

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