docs(codegen): document the remaining config options

[aryaemami59]

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.