Add a dedicated doc page for ValueConverter

[Shane32]

This issue participates in bounty program, see https://github.com/graphql-dotnet/graphql-dotnet/blob/master/BOUNTY.md

Bounty amount: $50

Tasks to be done:

• Consolidate existing documentation regarding ValueConverter
• Update documentation with additional documentation and samples
• Add documentation for IListConverterFactory and ListConverterFactoryBase, etc - see AOT compatibility for IListConverter; add migration notes #3932 and Add IListConverterFactory #3931
• Update site map

[ORIGINAL POST BELOW]

I think we need a dedicated doc page for ValueConverter. Currently, it is only mentioned in the Custom Scalar page. But it has a wider usage now.

Originally posted by @gao-artur in #3932 (comment)

[macco3k]

What would this page need to include?

[Shane32]

Not exactly sure. Probably we should work up an outline before any writing is done. Any ideas? Keep in mind that this PR applies to GraphQL.NET v8 specifically, as we are expanding the use of ValueConverter in that version.

We could start with looking at the existing documentation, of course. Maybe we should document the process that GetArgument / ToObject goes through to convert a value into the requested type, and ValueConverter's relation to it.

[macco3k]

I've not used ValueConverter myself, but from the little I can glimpse from that mention, it plays quite a big role in all the calls to GetArgument. Perhaps we should start with your suggestion and document what is already available.

[Shane32]

I added a bounty to this issue if you're interested in working on it.