Changes to navigation

[groganz]

The navigation could be changed as follows with each item pointing to the new category pages.

1. Getting Started (points to new category page at https://colossalai.org/docs/get_started)
2. Basics (points to new category page https://colossalai.org/docs/basics)
3. Features (points to new category page https://colossalai.org/docs/features)
4. Tutorials (points to new category page https://colossalai.org/docs/advanced_tutorials)
5. API (https://colossalai.readthedocs.io/en/latest/)
6. Blog