Add code examples to Basis' documentation

[Mickeon]

This PR attempts to add code examples to the Basis Variant type, which really needs some more detailing.

More knowledgeable users may already know what a Basis is and it feels like this class reference is written for them. For an introduction to the concept, this page is extremely dry, and it makes things look more complicated than they are in practice. _{I am also talking from personal experience here.}

The original intention was to overhaul the Basis class reference outright, but I was keen on splitting the PR in two. This is because I am not sure what would be a... good enough way to do this, and concentrated feedback from more "savvy" users and maintainers is very welcome here.

In particular, it would be nice to find a way to represent the Basis matrix in the examples in a way that doesn't look out of place or overly verbose.

One problem:
Each so-called matrix "column" is a Vector3, which when written tidily, forces the code examples to display them horizontally rather than vertically. It may confusing.

[Mickeon]

Addressed the above feedback. Also replaced every PI example with TAU.

[akien-mga]

Would be good to add the C# counterparts, CC @godotengine/dotnet.

[paulloz]

Would be good to add the C# counterparts, CC @godotengine/dotnet.

I can make a follow-up PR after this is merged.

[akien-mga]

Thanks!

[Mickeon]

Aww, the triple.