Documents: Add support for indexing and searching PDFs

[lastzero]

As a user with PDF documents in my library, I would like PhotoPrism to recognise and index them so that I can find and view them from the web interface.

• The first step is to add a new "document" media type to our backend. Additional changes will be required to find and index PDF files when scanning the library.
• Once the frontend component framework upgrade is complete, we can proceed with implementing a viewer (e.g. an iframe) to view the indexed documents from the web interface.
• Making the text searchable is beyond the scope of this issue, but can be implemented at a later time.

Sample Files:

• https://dl.photoprism.app/samples/Formats/Document/PDF/