Add link to symbols page from any relevant :doc output

[byorgey]

The Disco reference documentation has this page:

https://disco-lang.readthedocs.io/en/latest/reference/symbols.html

which can be helpful but is hard to find unless you know it's there. We should add a link to this page to the output of the :doc command for anything relevant. For example, something like this:

Disco> :doc /\
~/\~ : Bool × Bool → Bool
Alternative syntax: ~and~, ~∧~, ~&&~
precedence level 4, right associative

Logical conjunction (and): true /\ true = true; otherwise x /\ y = false.

https://disco-lang.readthedocs.io/en/latest/reference/logic-ops.html
https://disco-lang.readthedocs.io/en/latest/reference/symbols.html