Documentation

# Why

The repo is meant to provide an API for IPLD but it's currently undocumented, which makes its use harder (and maybe error prone)

# How

- [ ] Add docstrings to declarations
- [ ] Check the possibility of using [`doc-gen4`](https://github.com/leanprover/doc-gen4) to create a pretty documentation page

Review the repository's declarations to identify the public IPLD API that needs docstrings. Then investigate whether doc-gen4 can generate the requested documentation page; the work is done when the declarations are documented and the feasibility or resulting documentation page is recorded.

Written by the indexing model from the issue text.