[DOCS] Enhance .env.example with descriptive comments and links

[Adithyakp86]

Problem

The current .env.example file is a bare list of variables. New contributors may not know where to obtain these keys or which ones are critical for basic local development.

Proposal

Update .env.example to include:

1. Links to CoinGecko API dashboard (https://www.coingecko.com/en/api).
2. Instructions for obtaining Firebase config from the console.
3. Comments distinguishing between required and optional variables.

Scope

• Documentation

Assign this issue to me under the oscg