Docs: document lerna commands for local development

[saragerion]

Description of the improvement

To contribute locally to Lerna projects, developers need to run a number of commands to get their projects up and running locally.
Commands that need to be run include (but not limited to: lerna bootstrap).

Summary of the proposal

Edit the documentation related to local development to help developers and future contributors getting the project up an running.

How, where did you look for information

https://github.com/awslabs/aws-lambda-powertools-typescript/blob/main/CONTRIBUTING.md#dev-setup

Missing or unclear documentation

Dev setup section needs an update.

Improvement

Step by step guide with list of correct commands.

Related existing documentation

https://github.com/awslabs/aws-lambda-powertools-typescript/blob/main/CONTRIBUTING.md#dev-setup

Related issues, RFCs

#415
#115
#426

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