[WIP] Add an index tree, close #827

[bennuttall]

WIP - closes #827

[bennuttall]

@JamesH65 first draft of this is done - it is just the output of tree -D converted into a nested list with links. The titles could be improved as they're just the link names not in all lowercase. And we might be missing some pages which aren't README files in folders. Bit of a pain to automate as it's a little inconsistent.

https://github.com/raspberrypi/documentation/blob/827-index-tree/index.md

Not perfect but I think it's useful though! Certainly better than nothing.

[JamesH65]

If there is stuff that is inconsistent, we should probably fix that first, because I reckon we ought to be able to automate this.

[aallan]

In light of #1911 and the lack of progress in a long while I'm marking this PR as 7 day closure.

[aallan]

After consideration, I think this is superceeded by #1911 so closing.