Fix and expand Environment Variables setup instructions in Developer Documentation

[coderNini]

Summary

Several issues exist in the Environment Variables section: a filename error, missing command, vague API key instructions, an unclear page transition, and ambiguous phrasing around the app's root directory.

Proposed Changes

• Fix the filename: "Copy .env.sample to .env" should read "Copy .env.example to .env"
• Include the specific shell command to perform the copy (e.g., cp .env.example .env)
• Add a link to the Sandbox Admin account login page
• Expand API key creation instructions to specify: which scopes to select, how to name the key, which Redirect URI field to fill out, what all other fields mean and whether they require input
• Clarify the page transition between steps iv and v
• Provide clear instructions on formatting and organizing variables in the .env file (naming conventions, ordering preferences, etc.)
• Replace "In the root directory of Flextensions app" with "In the 'flextensions' directory" for clarity

Make changes in #408 (and refer to comments for further detail)