feat: Adds support of multi-language & custom 404 page

[frgfm]

This PR introduces the following modifications:

• made the whole website content language dynamic (enabling French & English)
• added flag icons to switch language in the navigation bar
• added explanations in the readme to add a new language
• updated gitignore to avoid pushing unwanted files
• adds a custom 404 page

Now the base URL is in french, and it has two subURLs /fr and /en referring to the digram of the target language.

Here is how it renders clicking on the French flag

And clicking on the English flag

All the content can be edited in the _config.yml.

Finally if someone tries to access an unknown URL:

Closes #4