Document `areequal` and `@areequal`

While looking through the docs, I noticed that the `areequal` and `@areequal` macros aren't currently documented, and are only briefly mentioned by example in the tutorials and the explanation of equality saturation. It'd be great if docstrings could be added for those methods, since I think they are one of the more useful analyses this package can perform.

(Thanks for this really cool package! I've been using it for some simply term rewriting tasks, and am looking to use it to write an equivalence checker for expressions in a fragment of epistemic modal logic -- hence my question about `areequal`.)

Start by locating the definitions of the `areequal` and `@areequal` macros, then read their existing examples in the tutorials and equality-saturation explanation. Add docstrings describing both macros and verify that the documentation clearly explains their use and purpose.

Written by the indexing model from the issue text.