TODO for nnue.md

[Sopel97]

• some results regarding net size (https://docs.google.com/document/d/1gTlrr02qSNKiXNZ_SuO4-RjK4MXBiFlLE6jvNqqMkAY/edit#heading=h.p7t1qfux64dc exps 84, 85, 86, 89)
• about data used to train the nets (filtering, mirroring and stuff)
• fix header links in doc
• make headings understandable without context
• consistent formatting (for feature set names, net arch names, etc...)
• backprop math
• linear layer with block sparse outputs forward (no results yet but it's an interesting direction)
• linear layer with sparse inputs forward (put some data about sparsity after CReLU)
• ways to reduce the network size with Half* (less king buckets, perspective mirror)
• either write something about weight pruning or remove the mention that it will be described later in the document
• revise implementation for sparse input FC (pad nnz indices so that only one loop is needed. use maddubs -> madd(ones, ...) implementation that processes 4 inputs at a time, with 8 bit weights). See https://pastebin.com/zDa7PbJh
• a better nnz indices implementation for the above, see On sparsity and the best way to compute indices of non zero elements. syzygy1/Cfish#204