revamp sigils section

[gfldex]

For now we just list the default types but don't really explain what that means.

• we don't explain the relationship between sigil-& and postcircumfix (and as such CALL-ME)
• we don't explain flattening rules for sigil-@ and list literals or link to /language/containers#Flattening,_items_and_containers
• we don't explain that sigil-$ will always create a container or link to /language/containers#Scalar_containers

It may be better to get rid of the table and link to the respective section in containers.