French locale added

I noticed you had a `config/locales/en.yml` file, so since I’m a native French speaker I added a French locale. Please note this is the first time I write an i18n file for a Rails app.

Some strings are missing from this file, and thus a few parts of the website are still in English.

Here are a couple notes on this translation :
* I voluntarily used English words for hard-to-translate concepts. For example, there’s no French word to describe what a webhook is, and most people use the word “README” in French instead of our equivalent, “LISEZMOI”.
* I used the word “évaluation” for “build”, because what you mean by “builds” is “documentation evaluations/tests” and there’s no really good French word for “build”.
* I didn’t understand the purpose of the line “Let's start a discussion, let's raise the visibility of documentation, let's talk.” Do you want to discuss _and_ talk? I translated this to something that roughly say “Let’s discuss, let’s raise the visibility of documentation”
* I translated both “inline documentation” and “code documentation” with the same “documentation de code”, because I don’t know any equivalent of “inline documentation” in French that doesn’t sound weird.
* A couple strings could be improved with more info, for example `shared.code_object_roles.object.without_doc` needs an article before `%{object_type}` but it depends on the word used for `object_type`.
Done. But strangely, when I do that, I lose the French translations in the browser.