Use popover for PHP class name explanation

[mbrodala]

Currently hovering a PHP class name with the php role yields a tooltip with a short explanation:

However, it would be better if this explanation was triggered via click/keyboard instead. For example using the (?) on the right as trigger.

Benefits:

1. Better accessibility by enabling keyboard access
2. Option to move the API/docs link into the popover; this allows for easier copy and paste of PHP class names without unintentionally opening a link.
3. Less "noise" in general since the explanation is really only shown on intent, never by accident.

[linawolf]

See also https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/Reference/ReStructuredText/Links/Api.html#links-api-fqn in the docs