General Docs Overhaul

[S-Haime]

This PR started as a 'scaffolding' PR, intended to provide the structure for updates. Due to how things went, this has turnt into a big update instead. This closes #174

It touches nearly all of the docs, it includes:

• Clarity edits and updates for roles and permissions. This should make the content usable for users, whilst we wait on some devdocs around roles and permissions.
• Work on using email templates and variables. It aims to make this part of Janeway more accessible to users and give them a clear idea of what is/isn't possible.
• There has been a lot of confusion from users around how to edit email templates and how variables work. This piece of work will hopefully help with this. We may need to test it / QA it with someone as it's one of the trickier parts of the manager interface.
• Slight rewriting of parts of the identifiers documentation, based on some of the gaps identified + user feedback. The DataCite elements still need additional work and documentation.
• Explanation of what the cache is and why someone may wish to clear it.
• Carousel configuration guidance. (New page.)
• Journal management landing page, to help users find the documentation they need. (New page.)
• Edits around indentation, capitalisation, punctuation and other formatting/copyediting details.
• A start on repository documentation