[#921] Edit the data viewer docs

Move the references to the resource_proxy extension in the text_preview
and pdf_preview sections into seealso boxes.

In the text_preview and pdf_preview docs themselves, don't say anything
about local vs remote files.

Change "can not" to cannot

In sections about individual extensions use singular eg. "this extension
is part of CKAN" not "these extensions are".

Change the reference of get_proxified_resource_url() to a :func:
reference, this doesn't make any difference currently but if we ever
added an API doc for the function to our Sphinx docs (e.g. via autodoc)
it would be hyperlinked to it.