docs: Add usage sub-section in docs

[dreamorosi]

Description of your changes

Following the changes introduced in #550 and this comment from @saragerion, this PR introduces a new short "Usage" sub-section under the "Getting Started" section of each utility's page.

The goal of this section is to give a very basic example of the library's usage (just import & initialisation outside of handler) and point out why the utility has to be instantiated outside of the handler, explaining also benefits of doing so.

This wording was initially under the SAM section because that was the first code snippet but given that this disclaimer is not SAM-specific it deserves its own section.

How to verify this change

Tracer

Logger

Metrics

Related issues, RFCs

#550

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• The code coverage hasn't decreased
• New and existing unit tests pass locally and in Github Actions
• The PR title follows the conventional commit semantics

Breaking change checklist

N/A

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[ijemmy]

I change the wording a bit to make it more accurate and concise. What do you think?

[ijemmy]

Suggested change

The `Logger` utility should always be instantiated outside of the Lambda handler. In doing this, subsequent invocations processed by the same instance of your function can reuse these resources. This saves cost by reducing function run time. In addition to this, `Logger` can also be aware of things like whether or not a given invocation had a cold start or not and inject the appropriate fields into the log.

The `Logger` utility must always be instantiated outside of the Lambda handler. In doing this, subsequent invocations processed by the same instance of your function can reuse these resources. This saves cost by reducing function run time. In addition, `Logger` can track cold start and inject the appropriate fields into the logs.