Documentation page for dates

[beganovich]

As of writing this, in the admin portal, we redirect people to official PHP docs when it comes to date formats. This must be improved by a dedicated page in docs.

Settings > Generated numbers > View date

What needs to be done:

• Introduction on date formats in general
• Common date formats

At the end of the article, we can include a link to the PHP docs themselves.

Technical details:

This definitely goes under "User guide" in docs. This is probably the right place to do it: https://invoiceninja.github.io/docs/advanced-settings/#generated_numbers