Update the content of the website

Atom community has surpassed its initial goal, and the content of the current website is out of date. We should market the organization as an independent community-driven effort, and we need to focus on the many features and packages offered.

The installation procedure has also changed. As of now, the installation of "atom-ide-base" would install everything automatically.

We don't need to mention the old ide-ui package and the history of Nuclide and Facebook. Especially, because in the future versions, we will not allow/recommend simultaneous running of ide-ui due to the conflicts and the bugs it creates for our packages [(example)](https://github.com/atom-community/atom-ide-debugger/issues/10#issuecomment-735370490).

Other than few minor things, the atomic packages cover all the things ide-ui provides and much more. The license files already cover the "possible" use of code from nuclide.