Improve "Register custom converters" page on project wiki (documentation)

[ADjQ]

Hello,

I was planning to implement a custom converter to my local version of sql2o but as I was checking the documentation to do so, I realised that the section regarding the registry of custom converters is a bit vague and confusing to understand.
It is quite outdated, as the recent versions of sql2o are compatible with Java 17 and newer (something that is also not detailed on the README but on the releases tab, even though I saw a pull request regarding such changes in the README), yet the current documentation details examples and procedures for older versions of Java.
Even if that part of the documentation reflects how to implement converters for older versions of the library, there is still missing info; someone detailed in the issue #306 that in version 1.6.0 you should also add --add-opens java.base/java.lang=sql2o to the VM run parameters, and that information is also not detailed on the current wiki page.

In general, the documentation should be updated to reflect the recent versions of the library, but that specific page is severely outdated and vague, and it would be a good idea to update it

[aaberg]

You are right. The documentation is lacking in many areas, including for custom converters, Java version compatibility, and others. Presently, there aren't many active contributors to Sql2o, and none are actively updating documentation.

Any contributions to documentation are highly appreciated :)

[aaberg]

I'll prioritize adding updated documentation for converters soon