Corpus form: explain uploading data from more applications

[lukavdplas]

This is about the manual page for uploading source data: https://github.com/CentreForDigitalHumanities/Textcavator/blob/develop/frontend/src/assets/manual/en-GB/uploading-source-data.md

This explains how to export data from a spreadsheet editor and Python in a compatible way.

Perhaps we could also add:

• Uploading data from OpenRefine. (Pretty straightforward, but some features/options are worth pointing out because they need to be set in a specific way, e.g. date formats or records mode.)
• Uploading data you downloaded from Textcavator. This should go without a hitch anyway, but it's nice to mention that it's an option.