Reformat documentation and contributing guide

[vbaconnet]

Some very minimal reformatting of the documentation and addition of the contributing guide (moved to doc/pages).

Additional changes are welcome! The index page could be improved. Perhaps the "Contributing", "Programming patterns" and "code style" can go under a common section for example :)

@timfelle I am marking this as a draft for now if you'd like to add some more things!

[timfelle]

Hey @vbaconnet,

I have completed a significant update to the pages. Could you have a look at it?

Summary:

• Restructure directories. This should make it easier to find stuff in the folder since we were having quite a lot of files at this point.
• Update the landing page, now lists the full table of contents for the documentation.
• Structured the documentation such that Neko Manual now includes 3 subpages, User guides, Developer Guides and Appendices. Each of which have their own sub pages.