docs: update the documentations to be consistent to each other

[Huerte]

Type

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

Overview

This PR updates the repository templates and documentation to follow open-source community guidelines. It simplifies instructions, fixes a self-referential link, and replaces em-dashes in both markdown files and CLI output.

Changes

• CONTRIBUTING.md: updated the list of PR template sections, fixed a self-referential link, and removed the completed pytest-cov task.
• README.md: replaced an em-dash with a new sentence to comply with project writing guidelines.
• CHANGELOG.md: replaced em-dashes with colons for formatting consistency.
• src/pygitgo/main.py: replaced em-dashes in CLI terminal success messages with colons.

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)