[RFC] Add 'append'

The following function:

```haskell
append :: Semigroup a => a -> a -> a
append = (<>)
```

I think it can be useful and beginner-friendly 🙂 Especially with more docs! Shouldn't break a lot of code and should be easy to move to append, since most of the `append` appearances on Hoogle are already for the types having the `Semigroup` instance.

Review the repository's current public API and documentation conventions for Semigroup operations; the issue names no files or tests. Done means deciding whether to add the proposed append function and documenting it consistently if accepted.

Written by the indexing model from the issue text.