Fix: Improve sidebar navigation

[unnati-jaiswal24]

fixed the bug. 🐛
Refference: doc📃 section navigation:
The changes which will now navigates to the correct ✅documentation .
index.md : Now ensures the main toctree, separates overview and tutorials .
overview.md : Updated its heading , and removed the toctree section as it was keeping hidden links .
conf.py: Added the max depth of headings similar to those of index.md.
This will now help user to find and jump to the part of documentation user wanted to read.

[quaquel]

Thanks for this PR. As pointed out in #2879, however, I am unsure whether there is a problem and whether this PR is necessary. I will await a response from @tpike3 to clarify exactly what the problem is, according to him.