docs: improve docs for UnstructuredFileConverter

[anakin87]

While UnstructuredFileConverter is powerful and quite used by the community,
our current docs and integration page are minimal.

I would like to expand the current docs

• specify/explain all the supported APIs: hosted - free and paid; local - via Docker (see also Unstructured: better support for hosted APIs #169)
• better show that the converter supports passing a directory and automatically converting files within it
• better show some of the supported file types

[dfokina]

@anakin87 When you have a chance to get to this issue, I have created a Notion page with some notes.

[anakin87]

@dfokina thanks!
Done.

When the docs are updated, I can take care of updating the integration page similarly...

[dfokina]

Updated the docs: https://docs.haystack.deepset.ai/docs/unstructuredfileconverter

[anakin87]

update integration page: https://haystack.deepset.ai/integrations/unstructured-file-converter