docs: improve sample.env comments for better developer onboarding

[Nouhaaouiti]

The sample.env file lacks sufficient comments to help new developers
understand what each environment variable does and where to get the values.

I would like to improve the comments by:

• Adding links to relevant dashboards (Auth0, Sentry, Algolia, etc.)
• Adding helpful notes for each variable
• Improving section organization

[camper-chan]

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