Change master branch to main

[dichotommy]

Discussed in https://github.com/meilisearch/documentation/discussions/1263

^{Originally posted by fharper January 6, 2021} Let's be allies and make this change that means a lot.

Here is a blog post that explain a little more why it's important, and how to easily do it. It will be a bit more complicated with automation, but we still should do it!