Improve documentation structure

#### The current picture:
The structure for the readthedocs documentation for EvalAI (https://evalai.readthedocs.io/en/latest/) is a bit hard to follow, The titles of different section as seen in the panel on the left look like random topics where two unrelated or two similarly titled topics are placed next to each other.

#### The improvement:
As the documentation covers a wide variety of topics, we should organize the topics into further sections with friendly names such as: **What is EvalAI?**, **How do I use it?** and **How to contribute?**. This is just one way, we can be more creative here.

For a visual idea of the sections, here is a snip from [Scrapy's documentation](https://scrapy.readthedocs.io/en/latest/), the sections and topics are excellently titled, and finding the required information is really easy:

Start by reviewing the current EvalAI ReadTheDocs structure and the left navigation, then compare it with the linked Scrapy example. Group the existing topics under clear sections such as project overview, usage, and contribution; the work is done when related topics are easier to find in the navigation.

Written by the indexing model from the issue text.