Index documentation

[MichaelMure]

I'm trying to grasp how the indexes in this repo work, what they are good for, the pro/cons of each, but I end up reading the code to try to figure out what's happening.

Would you mind writing a minimal doc for each explaining those? That'd really help.

[willscott]

There's a first pass at documentation at https://ipld.io/specs/transport/car/carv2/#index-format

In practice, we've ended up basically only using the 'MultihashIndexSorted' variant because we do want to be able to use the index for iteration of multihashes in addition to lookups. The tradeoff is that 'MultihashIndexSorted' is slightly larger but allows that, while 'IndexSorted' is slightly smaller but does not (because it doesn't include the multihash algorithms used, only the digests.)