Revise "hugo new project" (and the quickstart) and remove "hugo new theme"

[bep]

We have a quickstart that's not very quick and could be ... simpler. A lot have been written about that elsewhere, so I'm not going to elaborate.

• The quickstart has too many moving steps, and depending on an external theme is probably not the right choice to get started.
• A theme is also a project, so I don't see why we want 2 different commands for it. Also, the current hugo new theme file structure is a little too verbose for my taste. It works great as a documentation (showing all different layout types), but I suspect it comes off as a little bit daunting for new Hugo users. I suspect it would be better to e.g. add a all.html layout with some comments in it, possibly some links to the documentation.

I'm going to create a separate Go project where I'm going to prototype a new generator setup, but some ideas:

• No theme, and just Hugo native build tools, e.g. css.Build and js.Build etc.
• Some predefined enums for color palettes and layouts, e.g. hugo new project --colors pastel --layout blog-about (where pastel would be a set of CSS vars written to hugo.toml).

It would also been interesting/useful if the palette/layout config could be added to a section in hugo.toml for themes to respect, but that would need some more thinking ....