Issue 1913/docs add more datetime examples

[chwt]

Closes #1913

To have more examples of supported supported humanized datetime strings, added a file extracted from unit tests of the used datetime parsing dependency (https://github.com/bear/parsedatetime).

Questions/comments:

• I rather wanted to have as many examples as possible, but is the current list too much?
  • The file is located in a new directory docs/res/. Should it be moved somewhere else?
  • First, I had the grep command, which is used to extract the file, in the file itself as header. Then thought that it will rather confuse the documentation users. Currently it is part of the commit message. What would be alternatives?
• Please provide any suggestions for better phrasing of the added documentation change.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed. (docs only)