Add theory sections

[RastislavTuranyi]

Adds some theory around HODLR and the library, as we discussed. This is still in progress, but the basics are mostly complete, so please do have a look so that I can make corrections as soon as I am back.

You can also download the HTML pages from the build artifacts, if you'd like to see how this will be rendered in the end.

[manueldsdias]

Hi Rastislav, congratulations for the nicest documentation I've seen in a while! I like the way you explain things and I only found a few typos. Are you adding more to this or can it be approved?

[RastislavTuranyi]

Thanks! As you have pointed out, I was planning to fill out the why_hodlr.rst section, at least a bit, and I am also working on some diagrams for the looping stuff in the spare time. Additionally, I wanted to add explanations for the mathematical operations (i.e. how the HODLR-Vector, HODLR-Dense, and HODLR-HODLR operations work) in here, unless you wanted those elsewhere?

[manueldsdias]

Thanks! As you have pointed out, I was planning to fill out the why_hodlr.rst section, at least a bit, and I am also working on some diagrams for the looping stuff in the spare time. Additionally, I wanted to add explanations for the mathematical operations (i.e. how the HODLR-Vector, HODLR-Dense, and HODLR-HODLR operations work) in here, unless you wanted those elsewhere?

I don't have a strong opinion on this, you can place them where you or @haldaas feel makes most sense. I can read this section once you feel that it's ready.

[RastislavTuranyi]

Ok, thanks! If you (and Hussam) think the current stuff makes sense (both from what is written and where it is placed) and is headed in the right direction, I'll finish writing up the remaining stuff and let you know