chore(docs): update project's description

[dreamorosi]

Description of your changes

As described in #1289, we want to tweak the project's description to better express the focus of the project and disambiguate the fact that Powertools is not just about observability, but instead it's - more broadly - about increasing developer velocity around Serverless workloads.

Closes #1289

How to verify this change

See diff in the PR, or rendered files in the branch.

Related issues, RFCs

Issue number: #1289

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 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

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

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