Link to dynamic collections from collections page

[LeaVerou]

We have a page on dynamic collections, but it's under "Expressions": https://mavo.io/docs/expressions#mv-value
This makes sense from a concepts pov, but it makes it hard to find, since conceptually it seems more related to collections than expressions, yet there's nothing about it on the Collections page: https://mavo.io/docs/collections

We should add a pointer to it, that links to the relevant section in Expressions.