Add environment variable configuration

Description

This PR adds support for environment variable configuration in the Docusaurus project. Environment variables are used to store sensitive data or configuration settings that may vary between deployment environments. This allows for greater flexibility and security in managing the project's configuration.

Summary of Changes

Added a new section in docusaurus.config.js to define environment variables and their default values.
Updated relevant parts of the configuration to use the values from the environment variables.
Created a .env.example file to list all the environment variables used in the project, along with example values.
Added .env to the .gitignore file to prevent the actual .env file, which may contain sensitive data, from being committed to the repository.

Please review the changes and provide any feedback or suggestions.

Fixes #4.

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chunbaiseakuang / docusaurus-2

Add environment variable configuration #5

Description

Summary of Changes