Add new OPA errors docs for more parse error classes

[charlieegan3]

Now that we have #728, we link directly to the related error for a given parse error. This makes it more important to have pages for each of these errors at docs.styra.com/opa/errors.

Some more error types that'd nice to document:

• unexpected import path, must begin with one of: {data, future, input, rego}, got: foo
• unexpected eof token
• unexpected eof token: expected rule value term
• unexpected : token
• if keyword is required before rule body
• package expected
• found empty body not needed as not a thing in OPA v1
• rule name conflicts with built-in function
• expected ident