'Agtype NULL vs Postgres NULL' Section in 'Data Types - An Introduction to agtype'

[RC-002]

There is a heaading named 'Agtype NULL vs Postgres NULL' under 'Null' in 'Data Types - An Introduction to agtype' which is neither linked nor filled.
https://age.apache.org/age-manual/master/intro/types.html#agtype-null-vs-postgres-null.

[muneebkhan4]

I have created a pull request to resolve it. #128