v2.0 Remodelled backend

Big Book of R was originally just a plain text file where the books were directly added in markdown.

This was great for simplicity, but had serious limitations. It was hard to count books, hard to alphabetise them and impossible to do almost anything else with the collection.

I've updated how this works. Now I have captured all the books in a google sheet. The markdown is then generated by a couple of nested for loops and the content rendered with markdown/bookdown as before. Books are now alphabetised per chapter, and adding a book to multiple chapters is easier too.

The presentation of book entries is a little neater now and consistent. What I'm excited about is that having the book entries stored in a machine-readable format now make sit easier to build other automations to aid discoverability of these books e.g. a twitter bot that highlights books from the collection.