Add Self-hosted Umami analytics setup

[synan-mannan]

Description

This pull request adds a complete setup guide for self-hosted Umami analytics.
It includes detailed instructions for environment variables, Docker deployment, PostgreSQL configuration, dashboard access, and website tracking.
The README has been enhanced to provide a step-by-step guide, making it easier for contributors or the organization to deploy Umami for privacy-first analytics.

Related Issue

References Issue: Set up analytics tracking tool #114

Motivation and Context

The project lacked a documented setup for self-hosted analytics, which is essential for tracking user behavior and traffic without relying on third-party services.
This enhancement ensures the organization can deploy Umami quickly, maintain visitor privacy, and have complete control over the analytics data.
It improves documentation quality, reduces setup errors, and makes contribution easier for new developers.

How Has This Been Tested?

• Verified the README renders correctly on GitHub.
• Confirmed that the steps described are consistent with Umami's official documentation.
• Tested the Docker commands in a local Ubuntu environment with Node.js ≥ 18.17 and PostgreSQL ≥ 12.14.

Screenshots or GIF (In case of UI changes):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• [ x ] Documentation / Enhancement (non-breaking change)

Checklist:

• [ x] My code follows the code style of this project.
• [ x] My change requires a change to the documentation.
• [x ] I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

! Note: I added new documentation describing the analytics setup; existing docs were not modified, but documentation coverage was extended.