unify partial deep type & improve `defaults` function examples

- [ ] unify `PartialDeep` to use
```ts
export type PartialDeep = O extends Record
? { [K in keyof O]?: PartialDeep }
: O
```
- [ ] `export` this type as well
- [ ] make sure that `merge-anything` works correctly with the PartialDeep type I end up using

[DOCS]
- [ ] rewrite the documentation for the defaults... functions to use `PartialDeep`
- suggest to use `merge()` function for cases with nested objects
- [ ] explain how to be re-use modules with config in other modules with other where filters