RANGER-5353: Introduce Material for Mkdocs for Apache Ranger Docs

[kumaab]

What changes were proposed in this pull request?

• Initial design with Material for Mkdocs for Ranger Docs website.

• Material for MkDocs is a modern documentation framework built on MkDocs, a static site generator for Markdown files. GitHub Repo: https://github.com/squidfunk/mkdocs-material, more details in jira.

• README.md contains instructions to build and host the doc site on a local machine.

• Only the landing page uses custom HTML, CSS and Javascript.

How was this patch tested?

The new CI job: docs is successfu, CI runs:

• https://github.com/apache/ranger/actions/runs/20739044492
• https://github.com/apache/ranger/actions/runs/20766148556

Preview

• The changes in this PR are available for preview here: https://apache.github.io/ranger/
• Available to use in light/dark modes, with search functionality built in.