docs/policy-reference: Update Rego EBNF grammar (#4955)
docs/policy-reference: Update Rego EBNF grammar (#4955)

Includes:
* Add support for membership and parenthesized expressions
* Membership rule now uses `term` for both of its LHS operands
* `rule-head` and `rule-body` changes to enable the future `if` syntax `rule if something`
* Add support for unary expressions, i.e. unary minus (`-input.x` or `-(input.x)`)

Signed-off-by: Pavel Odvody <2375fa63e2908bb2df59f606483a3aa92321db7a@redhat.com>
open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa