Improve docs about Metadata Filtering

[Amnah199]

Is your feature request related to a problem? Please describe.
The current documentation for Metadata Filtering lacks structure and is incomplete, making it difficult to understand. As we are also disabling legacy filters, our documentation should be clear.
https://docs.haystack.deepset.ai/docs/metadata-filtering

Describe the solution you'd like:
The documentation should include a decent explanation of available filter keys and operators. We can also include clear examples covering specific use cases such as nested filters, to demonstrate the correct syntax and usage.

[dfokina]

https://docs.haystack.deepset.ai/docs/metadata-filtering