Update doc based to reflect frontendchanges

[maudetes]

Close https://github.com/etalab/data.gouv.fr/issues/399.

Update doc to reflect frontend changes (following changes to udata3).

Additional changes:

• Updated some links that were down, better links can surely be found.
• Added schema description when uploading resources, help is welcome to improve it. We could also document schemas in Télécharger une ressource.
• Added MIME type for catalog pages.
• Added some warning but didn't update doc for pages that may change a lot in the near future (open for discussion).

[agarrone]

Thank-you ! I plan to set a short sprint on doc and the guides in the following month ;)