KQL Documentation

[mmattel]

Document the new and very powerful query language.

owncloud/ocis#7043 (enhancement: Keyword Query Language (KQL) search syntax support)
owncloud/ocis#7263 ([full-ci] enhancement: add support for natural language kql date ranges)
owncloud/ocis#7455 (add last modified filter chip)

https://github.com/owncloud/ocis/pulls?q=is%3Apr+kql+is%3Aclosed

@fschade fyi, as discussed

[micbar]

Currently I see this DEV docs only.