New section generator

[rg-wood]

Create new pages via an npm script, i.e. npm run new:section. This should:

• Create a new folder with the slug of the given title as folder name, i.e. chapter2.
• Create an index page for the new section, i.e. chapter2/index.md.
• Give it an index equal to the index of the last section +100, i.e. 200 for chapter 2.

Hygen is probably a good candidate for implementing this.