[Docs] Add developer documentation like in CSV.jl

I really like the developer documentation in [CSV.jl](https://github.com/JuliaData/CSV.jl/blob/main/src/README.md).

Would it be of interest to add something similar to Arrow.jl?

It took a week of starring at it and reading the Arrow IPC specs to understand what’s going on, so I’m probably not the best person to write it.
But I’d be happy to make the first draft.

Start by reading the developer documentation in CSV.jl's src/README.md and the Arrow IPC specifications referenced in the issue, then inspect Arrow.jl's existing documentation structure. Done means producing a comparable developer-focused first draft that explains how Arrow.jl works.

Written by the indexing model from the issue text.