Improve Hash Object Docs

[librasteve]

Problem or new feature

Currently, there is no link in the docs from the "Non-string keys / Object hash" section: https://docs.raku.org/language/hashmap#Non-string_keys_(object_hash)

to/from the class ValueObjAt section: https://docs.raku.org/type/ValueObjAt

Suggestions

This proposal is to cross link these sections so that there is advice on the best way to make a custom class with the intent to use that class objects as hash keys, in sympathy with this statement... So, the correct way of using a class as a key would be to set all attributes to be immutable, and override WHICH to return a ValueObjAt: https://docs.raku.org/type/ValueObjAt