Recursive URL loader (#6455)
We may want to process load all URLs under a root directory.
For example, let's look at the [LangChain JS
documentation](https://js.langchain.com/docs/).
This has many interesting child pages that we may want to read in bulk.
Of course, the `WebBaseLoader` can load a list of pages.
But, the challenge is traversing the tree of child pages and actually
assembling that list!
We do this using the `RecusiveUrlLoader`.
This also gives us the flexibility to exclude some children (e.g., the
`api` directory with > 800 child pages).
