doc: add installation instruction

[bagustris]

Description

Add an instruction to install using pip.

Motivation and Context

No installation instructions have been given on the installation page. In the Github readme page, I follow Installation link, but only Packaging status is given.

How Has This Been Tested?

pip install git-cliff

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)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[welcome]

Thanks for opening this pull request! Please check out our contributing guidelines! ⛰️

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.85%. Comparing base (e85888f) to head (2278fb7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #982   +/-   ##
=======================================
  Coverage   42.85%   42.85%           
=======================================
  Files          21       21           
  Lines        1734     1734           
=======================================
  Hits          743      743           
  Misses        991      991           

Flag	Coverage Δ
unit-tests	42.85% <ø> (ø)

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[orhun]

Hello! The instructions are already available here: https://git-cliff.org/docs/installation/pypi

But maybe we can rename PyPI to pip for making it more clear.