Consolidate Examples: memtable.rs and parquet_multiple_files.rs

[alamb]

Is your feature request related to a problem or challenge?

• Part of [EPIC] Improve examples to make them easier to navigate #11172

The large number of small examples in DataFusion leads to multiple problems:

• Makes it harder to find the relevant examples
• Unnecessarily large amounts of disk space on a full build (each example binary takes 100+ MB)
• Larger cargo t time (each binary must be linked / created)

Thus I think the fewer examples we have the

1. easier it will be to find the relevant examples
2. faster our dev experience will be

Describe the solution you'd like

I would like a single file with common SQL based examples where we can consolidate other files (like to_date.rs, etc)

• Consolidate the contents of https://github.com/apache/datafusion/blob/main/datafusion-examples/examples/memtable.rs
• And https://github.com/apache/datafusion/blob/main/datafusion-examples/examples/parquet_multiple_files.rs

Then update the readme page:

• https://github.com/apache/datafusion/blob/main/datafusion-examples/README.md

Describe alternatives you've considered

No response

Additional context

I think this is a good first issue as the task is straightforward and doesn't require deep understanding of the DataFusion codebase