add: Table of Contents to each chapter (2-12) using mdbook-toc

[thelissimus]

It is quite difficult to navigate back and forth without the table of contents, considering that the book is written in 12 big one-page chapters.

This PR uses mdbook-toc to generate TOC. The reason of using it is to prevent writing TOC manually - to prevent the chapter names and TOCs getting out of sync. It is added to both GitHub Pages and Netlify deployment pipeline environments. If mdbook-toc dependency is redundant the PR can be re-worked to remove mdbook-toc and add TOCs manually.

Only the GitHub Pages deployment pipeline tested (due to having no access to leanprover Netlify) and it is working perfectly. The demo deployment is available at: https://thelissimus.github.io/theorem_proving_in_lean4/