README and Guides are missing docs for `buffer(policy:)`

I think the README file should be include a comprehensive table of contents to this library, however at least `buffer(policy:)` is currently not listed there, and there is no DocC chapter to be found under [Guides](https://github.com/apple/swift-async-algorithms/tree/main/Sources/AsyncAlgorithms/AsyncAlgorithms.docc/Guides) either.

Start by inspecting the README and the Guides directory under Sources/AsyncAlgorithms/AsyncAlgorithms.docc, then locate the documentation for buffer(policy:). Done means the API appears in the README's library contents and has a corresponding DocC guide entry.

Written by the indexing model from the issue text.