New page generator

[rg-wood]

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

• Create a new page with the given title as main header and slug as file name, i.e. new-page.md with title # New Page.
• File it in the specified book section (i.e. chapter2/title.md)
• Give it an index equal to the index of the last page in the specified section +1 (i.e. 202 for a new page in chapter 2).

Hygen is probably a good candidate for implementing this.