Add: add comprehensive Gmail API documentation and polling setup guides (#45 #46)

[aperezmarquez]

This Pull Request introduces the necessary documentation to set up and maintain the Gmail Polling System. It provides a clear, step-by-step roadmap for configuring the Google Cloud environment and handling OAuth2 authentication.

Changes Included:README.md:
Updated to serve as the main entry point, providing a high-level overview of the polling logic, installation steps, and links to specialized guides.

• google_cloud_setup.md: A detailed walkthrough on creating a Google Cloud project, enabling the Gmail API, and configuring the OAuth consent screen with the required scopes (gmail.readonly and gmail.send).
• token_generation.md: Instructions for generating the token.pickle file. It emphasizes the requirement to run the authentication flow locally to allow browser interaction for user login.

Key Requirements Addressed:

• Documentation of the Authentication Flow: Explains the redirection to the browser, the "Test User" login requirement, and the granting of permissions.
• Environment Setup: Outlines how to install dependencies in a local venv and execute the generation script outside of a Dockerized environment.
• Security Best Practices: Added warnings regarding the exclusion of token.json and token.pickle from version control.