[update] several edits

[gsilvapt]

I did some changes to the layouts and added a few guides. It should solve issue https://github.com/okfn/handbook/issues/90, https://github.com/okfn/handbook/issues/89 and small bits of https://github.com/okfn/handbook/issues/87.

If someone could provide some feedback, I'd appreciate that

• [ ] Images are not displaying: Probably a poor use of Jekyll's assets
• [ ] Current structure does not please me - have to think about how to improve it
• [ ] Forgot to add how to make Pull Requests in working-with-git.md to this PR
• [ ] Missing table of contents (don't know how to do it)
• [ ] Menu needs updates (as per https://github.com/okfn/handbook/issues/91)

[rufuspollock]

@gsilvapt really good work. Some comments inline. General comments:

• More meaningful commit messages e.g.[update]: several edits does not tell me much suggest: [data][xl]: add several data guides and move all patterns under data-guides/ (note use of "size" descriptor for commit in second bracket [xs,s,m,l,xl] are the options).
• Slightly less in a single PR would be good -- at least at this early stage -- so I can review, we can make any adjustments and then do the bigger item.
• I am not sure about relocating all the patterns stuff. This is the kind of thing I really recommend discussing in an issue before just doing. Right now I'll merge but we may need to think about moving it back.
• All the checklist "issue" items might be better off in a specific issue (given the number and substance of them)