Use GitHub PRs to work on the standard while keep GitBook to publish it

[lecoqlibre]

I think it would be better to use GitHub PRs. Changes will be visible as usual in the "Pull requests" section of the repo.

@RaggedStaff pointed out that if we use GitHub we will have to write the markdown by hand and we won't have the GitBook preview feature anymore.

GitHub offers a basic markdown editor and preview mode, maybe less powerful than the one from GitBook.

I think as developers we should be able to write a bit of markdown.

On the other hand, GitBook has some downsides:

• we need to maintain an other team list with rights and privileges we already have on GitHub
• we have to set merging and reviewing rules we already have on GitHub
• we have to publish a public link to a change request somewhere
• public comments seem not possible
• it is a second place where we can be tagged in comments and might be hard to follow

I propose we use GitHub to work using PRs and we keep GitBook as the publishing system.

@datafoodconsortium/standard_maintainers