docs: add login guide with screenshots for gitgo user login

[Huerte]

Type

• Bug fix
• New feature
• Documentation
• Refactor / internal

Overview

Added a proper login guide with screenshots for gitgo user login. The README only had a short description. New users kept getting confused on the GitHub side so i wrote a full step-by-step guide covering the whole flow.

Changes

• docs/login-guide.md: new file. covers all 7 steps from running the command to verifying the connection. includes screenshots for each step.
• README.md: updated the login usage section to link to the new guide.

How to Test

1. open docs/login-guide.md and read through the steps.
2. check that the link in README.md points to it correctly.

Checklist

• I tested my changes locally and they work
• I updated CHANGELOG.md under the [Unreleased] section
• I updated README.md (if I added or changed a command)
• I added or updated tests for my change (if applicable)
• My change does not break any existing commands (if it does, describe the impact in the Overview)