chore(docs): updated layer info

[dreamorosi]

Description of your changes

As discussed in the linked issue, at the moment users opting for using the Lambda Layers published by Powertools didn't have clear instructions on how to deal with their local environment and dependencies. While this might be obvious for more experienced developers, there might be some customers that need a refresher, so this PR adds a section that clarifies this.

This PR closes #1357.

How to verify this change

See rendered

Related issues, RFCs

Issue number: #1357

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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