Add details regarding abstract and virtual functions in traits

[Oni-giri]

Hi!

I noticed that the docs didn't include any mention of virtual and abstract functions in traits, which can be overridden in the implementation contract. The virtual keyword for functions can be seen in an example in the Tact by Example website, for instance.

This is a great feature of Tact, so I think it's worth it to include it in the official docs! I also added references to it in other pages which were relevant.

[anton-trunov]

@Oni-giri Thanks for your contribution! Let's fix a couple of nits and we are good to go

[anton-trunov]

@novusnota check my recent changes please