Consider a way to make the extension configs extendable

Currently mdbook allows preprocessors and renderers to define their own config settings in their corresponding configuration table. For example, mdbook-linkcheck has a bunch of settings. However, these settings share the same namespace with mdbook itself which has settings like command, optional, renderers, before, and after. Each time mdbook adds its own configuration setting, there is a risk that it can conflict with existing extensions.

I think it would be good to consider a different structure to allow mdbook to add new settings without potentially interfering with existing extensions. This would allow new settings to be added in a backwards-compatible way.

I don't really know what that would look like. It could be some different sub-table like output.name.settings?

Start by reading how mdBook parses configuration tables for preprocessors and renderers, then compare the built-in keys with extension-defined settings such as those in mdBook-linkcheck. Done means proposing and documenting a configuration structure that avoids namespace conflicts while preserving a clear path for backwards-compatible additions.