docs: update docs for global variables page and security

[lucaseduoli]

This pull request includes several updates to the documentation, focusing on the configuration of global variables and the addition of security best practices. The most important changes include the consolidation of environment variable documentation, the introduction of a new security best practices guide, and updates to the sidebar for better navigation.

Documentation updates:

• docs/docs/Configuration/configuration-global-variables.md: Consolidated environment variable documentation, added a section on default environment variables, and removed redundant security information. [1] [2] [3] [4]
• docs/docs/Security/best-practices.md: Added a new guide outlining security best practices for deploying and managing Langflow, including secret management, database security, deployment security, and development security.

Navigation improvements:

• docs/sidebars.js: Updated the sidebar configuration to reflect the new structure, including the addition of the "Security" category and the new "Security Best Practices" guide.