Improve README page and Getting Started page. (#4145)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4145

Minor improvements:

- Link directly to the Getting Started page from README.md. It's always a few clicks away.
- Besides the colab notebook, also explicitly mention `pip install executorch`

Reviewed By: kirklandsign

Differential Revision: D59329875

fbshipit-source-id: 176780e9320500a947b47d5000a1c32c1f50a3c6