Improve the documentation about date formats configuration

[sequba]

Description

There are several issues about it:

• API reference for dateFormats and parseDateTime is not clear enough about what is possible
• '.' is also a valid date format separator (not mentioned in the docs)
• Improve https://handsontable.github.io/hyperformula/guide/date-and-time-handling.html - describe better why it's necessary to provide a custom parseDateTime function and make sure the guide is accurate in every detail.

Links

https://handsoncode.slack.com/archives/C031GBX62SE/p1648731737807499 (internal discussion)

Type of issue

• [x] Missing documentation
• [x] Error in the documentation
• [ ] Request for a demo
• [ ] Request for a tutorial, guide etc.
• [ ] Other issues, suggestions or ideas

[AMBudnik]

Closed as released in version 2.1.0.