docs(codegen): document the remaining config options

Part of #5368.

Ten of the 27 options in src/types.ts are absent from the docs page: unionUndefined, encodePathParams, encodeQueryParams, includeDefault, mergeReadWriteOnly, useUnknown, esmExtensions, exportAllSchemas, and prettierConfigFile.

Also missing: the supporting types block, and a note that the CLI resolves config paths relative to the config file.

Fixed by #5367.