docs: add mkdocs to Documentation section

[AkashGowdaNC]

Summary of Changes

• Added mkdocs to the ### Documentation section in README.md.
• Inserted the entry in alphabetical order directly before mkdocs-material.

Motivation

mkdocs is one of the most widely used static site generators for Python documentation, and mkdocs-material (which builds directly on top of it) is already listed in this section. Adding mkdocs provides developers with the foundational tool reference.

Specifications

• PyPI Name: mkdocs
• Canonical URL: https://github.com/mkdocs/mkdocs
• Description: Fast, simple and static site generator geared towards building project documentation.

Checklist

• Uses lowercase PyPI package name (mkdocs)
• Follows standard formatting: - [name](url) - Description.
• Description starts with a capital letter and ends with a period
• Placed in correct alphabetical order