[Fift > Constants, Variables, Words] Missing crosslinks

[jeshecdom]

Add link at following place, once page for basic values (#1468) is merged into main:

Internally, the word variable creates an empty box (link to boxes section)

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

For more details on blocks/execution tokens and words ' <word_name> and execute, refer to the Blocks Section in the Control Flow page. (link to blocks section)