[Website] Add pricing page
## Goal

To improve the sponsors engagement, we need to add a page that display the different tiers. Ts.ED stay free and open-source. The main idea is to push premium offers like "dedicated channel for company" etc... 

Rules:

- This page must be display when a user click on the slack link.

It let the developers to choose a tiers:

- The free tiers redirect the user to the slack (maybe a delay before enable button can be added)
- The others tiers redirect the user to github sponsors
