[BUG] Missing core gfql readthedocs docs

[lmeyerov]

Describe the bug

The docs for n(), e(), e_forwards(), and e_backwards() were not findable on the readthedocs:

• from chain() + the table of contents, unclear that those are the ones to look at
• what their parameters are
  • ex: that to_fixed_point is basically [e:*] in cypher (arbitrary hops)
  • while hops=k is for a fixed max (is that max, or exact?)
  • how e(source/destination_node_match=) works differently from doing a n(match=)
  • query=

[lmeyerov]

Realized this may relate to #536