Create better loading indicator experiences in serverside examples to enhance demonstration

In the `serverside-pagination` example, we have some sample means of adding loading indicators. Loading indicators are a common and important part of the serverside experience, so we should have some better examples here so that other devs can work from good ideas, and we can keep full-fledged loading indicator options out of the core library (E.g. https://github.com/gregnb/mui-datatables/pull/958).

- [ ] Create more realistic, good UX loading indicator example(s) in the `serverside-pagination` example.