docs: multiple minor improvements

[dreamorosi]

Summary

Changes

Please provide a summary of what's being changed

This PR makes a number of minor changes to the docs in a few pages. Specifically:

• Added the newly added environment variable (POWERTOOLS_METRICS_ENABLED) in the main page & feature page
• Added a section dedicated to the POWERTOOLS_DEV env variable in the main page, similar to the Python docs
• Updated the Lambda layer code snippet for Amplify to use Gen 2
• Removed the banner at the top of the page for the v1 EoE which was now 3mo old
• Removed the "new" marker from the workshop link (1y old) and the Parser section (6+mo old)
• Added a section in the Metrics utility page that explains how to use a custom logger,
• Updated the "testing your code" section with details on the POWERTOOLS_METRICS_ENABLED env variable
• Updated references to Middy.js to specify we support version 4 to 6
• Fixed deep links in the main and upgrade pages that were using old heading texts

Please add the issue number below, if no issue is present the PR might get blocked and not be reviewed

Issue number: fixes #3386

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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