Bug: Create consistecy for GitHub-related terms

[adiati98]

Describe the bug

Currently, there is no consistency when using some of GitHub-related terms, such as README and Contributing Guidelines.

Suggested Solution

Create consistency by following GitHub's convention for README and CONTRIBUTING as follow:

README

• Use README to refer the README in general.
• Use README file to refer the README.md file.
  https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-readmes

CONTRIBUTING

• Use contributing / contribution guidelines (all lower case) to refer the guidelines in general.
• Use CONTRIBUTING file when referring to CONTRIBUTING.md.
  https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

GitHub Actions

• Use GitHub Actions when refering to GitHub Actions.
• Use action(s) (lower case) when referring to general actions.
  https://docs.github.com/en/actions/learn-github-actions/understanding-github-actions#actions

YAML

• Use YAML when referring to the language and the file.
  https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms

Note: This will be an open issue, be fixed gradually, and be closed when all chapters are checked.

Originally posted by @adiati98 in #25 (comment)

Steps to reproduce

1. Go to each chapter files.
2. You can see some inconsistencies.

Browsers

No response

Additional context (Is this in dev or production?)

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

Contributing Docs

• I agree to follow this project's Contribution Docs