[Journals] Remove/reformat author guidelines page without journal specification
I find that there's an explanatory section of author guidelines in the submissions page (https://scipost.org/submissions/author_guidelines) which is duplicated over when looking at a specific journal (e.g https://scipost.org/SciPostPhys/authoring).
Furthermore, the first one is missing associated templates and "must-contained elements", leading to a malformed page: