For UTF-8: SQLSTATE[HY000] [2019] Unknown character set

A configuration instance is not strictly required by [DriverManager::getConnection](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/DriverManager.php#L139).

As an upside, it doesn't add an extra warning for people that use Psalm/PHPmd when starting off from the documentation example. As the Configuration class has an `@internal` block those tools threat it as the other meaning for `@internal`, as in class that is [internal to the library and not for public use](https://docs.phpdoc.org/references/phpdoc/tags/internal.html).

Gitter has been deprecated in favor of Slack for all Doctrine purposes.

revert back MariaDb1027Platform

Fixes #3121

Conflicts:
	docs/en/index.rst

See discussion at https://github.com/doctrine/doctrine2/issues/7187

It seems to be the case in the rest of this document.

Fixes #2760

One class missed and one non-existing in example at "Supporting Other Databases" page.