Update Docs

[willtebbutt]

This updates the docs so that we actually discuss the ColVecs / RowVecs approach to handling multi-dimensional inputs.

Specifically:

1. api.md has been re-vamped. More context added. This is the section that requires most extensive review.
2. userguide.md now has examples with ColVecs, RowVecs, and Vector{<:Real}. Also discussed the relationship with obsdim.
3. The README has been tidied up now that we don't need to do the whole reshaping to get a N x 1 matrix thing, and to use composition to do transformations. I've also removed the reference to using a BLAS backend, because we do that.
4. Improved docstrings for RowVecs, ColVecs, and MOInput.

[willtebbutt]

I'm making another pass over the API docs at the minute, so consider holding off reviewing that bit for now.

[willtebbutt]

Thanks all for the detailed feedback.

My plan is to

1. resolve all items that do not pertain to the structure of the docs
2. once that's done, shuffle stuff around to make a cleaner split between user guide, api, and a design notes. Thanks @st-- for pointing out that this would be a good thing to do. When this is done, I'll solicit another round of feedback.

[willtebbutt]

@st-- I've moved the bits of the API docs that are really more of a design discussion into design.md. Please let me know what you think :)

[devmotion]

Sorry, I missed your message on Slack. Can you add the "Design" section of the docs to the sidebar? It seems to be missing there. Otherwise I am happy with the PR, it is a major improvement over the current state and we can always improve it if needed. Feel free to merge once the sidebar is fixed.