Add support for office documents, as in the MSOffice and LibreOffice suites

[brainchild0]

Having investigated support within Nextcloud for interacting with office documents of various sorts, such as the various types from the MSOffice and LibreOffice suites, I have understood that presently the two available solutions are Collabora and OnlyOffice.

Both follow the same model of clients interacting with server instances outside of the Nextcloud server, and both such servers require vastly more resources than are available or necessary for the smallest Nextcloud instance.

While Collabora and OnlyOffice may be suitable at a scale for particular needs of collaborative editing, in a very large number of cases, support for office documents limited simply to viewing would be immensely valuable.

However, currently, no viewer application is available supporting such documents.

Considering the means that such an application may be achieved, the strongest candidate has seemed to be conversion of the source document to a PDF document that may be served to the client from a request.

If such a design were pursued, then it would seem less onerous simply to integrate such functionality into PDF Viewer.

An idea that comes to mind takes the form of generalizing the functionality of PDF Viewer to support viewing any document that may be converted to a PDF document with the help of a driver.

A driver for office documents would seem to be feasible through, for example, the project suite PHPOffice, a set of libraries totaling 10 megabytes in code size that supports conversion to PDF from word processor, spreadsheet, presentation, diagram, and project document format of either MSWord or LibreOffice suites.