Restructure documentation for custom converters

• Add a section "Customizing type mappings" to the "Data type mappings" section of all the connector docs (as seems to be missing in general); this should spend a sentence a long the lines of "If the provided type mappings are not sufficient for your use case, you may consider implementing a custom type converter. See "link to the page on converters" to learn more about this.
• Add the property to the table with all connector options, also linking to that central page

Ideally this should be implemented as fragment(s) like signalling.