Validator: make github field required for block_content examples

[Laren-AWS]

Currently, the github field is optional for examples, but the downstream effect of this is that WRITEME omits the example from the README, which is not intentional.

For the root repo (possibly under strict mode?), we should require this field at least for block_content examples, and possibly for all examples so that they are correctly included in the READMEs.

It is a valid case to omit this field when the source examples are in a tributary that's stored somewhere inaccessible, so we must allow a switch for this to optional.