Add Attribute linking

[haszi]

This PR adds support for attributes in the format described in this comment and generates links to the documentation pages of known predefined attributes.

Edit:
As a side note, there are 129 additional attributes in php-src stub files that are not documented in doc-en.

[kocsismate]

Do you plan to add support for displaying other types of attributes? Or is there any technical limitation?

[kocsismate]

gen_stub.php counterpart: php/php-src#14270

[haszi]

Do you plan to add support for displaying other types of attributes? Or is there any technical limitation?

This PR supports 'known' attributes on method and function parameters. 'Known' in this context means an attribute that has been declared in the documentation with an ID in the class.attribute_name format (all current predefined attributes are supported).

As far as this PRs limitation, this implementation doesn't support namespaced attributes and does not support linking an attribute on a return type either. It wouldn't be hard to add these though.

[Girgias]

LGTM thank you!