Fix: Update GitHub Actions workflow and enhance documentation

[skhatri]

This commit addresses a build failure by updating the GitHub Actions workflow and includes the previously prepared documentation enhancements.

Workflow Updates:

• Switched to using the peaceiris/actions-mkdocs-gh-pages@v4 action for a more robust and maintained deployment process.
• Configured the workflow to use Python 3.11.
• Dynamically creates a requirements.txt for MkDocs dependencies, including mkdocs-material>=9.0.0 and other necessary plugins.
• Enabled the enable_pdf_export: true option in the deployment action to handle dependencies for mkdocs-with-pdf (e.g., WeasyPrint).
• Set force_push: true for deployment, consistent with previous behavior.
• Workflow now also triggers on pull requests to main for pre-merge checks.

Documentation Enhancements:

• Significantly revamped index.md, whyoca.md, and whatisit.md to improve readability and visual appeal.
• Updated mkdocs.yml to enable several Material for MkDocs features and PyMdownX extensions (admonitions, Mermaid diagrams, tabs, icons, etc.).
• Replaced custom span-based styling with theme-native admonitions and icons.
• Restructured content using admonitions, added icons, and placeholders for Mermaid diagrams.
• Clarified analogies, improved descriptive text, and added calls to action.

These changes aim to ensure successful documentation builds and provide a more engaging, navigable, and effective documentation site for Ops Catalog.