[Fift > Basic Values] Missing crosslinks

[jeshecdom]

Add link at following places, once Types (#1426) is merged into main:

As described in the Types page,

A Box is a location or container that can be used to store exactly one value of any Fift-supported type

Add link at following place, once page for control flow (#1541) is merged into main:

Certain operations, like conditional execution of blocks and loops with exit conditions,

Add link at following place, once page for list of common words is available:

For a list of words for manipulating slices and reading their contents, see the list of common slice words page.