Write librarian documentation for "Web Books" & Import Scheme for adding provider links

### Problem

### A clear and concise description of what you want to happen
Write librarian documentation for adding web book provider links + link to the help text on the edit page. This includes writing guidelines for what we want to be included (put somewhere nice + link to from edit page + post in slack + also ping Onno).

### Proposal & Constraints

_No response_

### Leads

### Related files

### Stakeholders

### Instructions for Contributors

- Please [run these commands](https://github.com/internetarchive/openlibrary/wiki/Git-Cheat-Sheet#working-on-your-branch) to ensure your repository is up to date **before** [creating a new branch](https://github.com/internetarchive/openlibrary/wiki/Git-Cheat-Sheet#making-changes-and-creating-a-pull-request) to work on this issue and **each time after** pushing code to Github, because the pre-commit bot may add commits to your PRs upstream.