docs: add guides for custom scalars

[sarahxsanders]

Adds two guides for custom scalars:

• When and how to use them
• Best practices and testing

This is a part of the effort to expand GraphQL.js documentation

[saihaj]

we can share community resource for GraphQL JS Scalars

• https://the-guild.dev/graphql/scalars
• There was also this spec that started but seems to be no longer maintained https://github.com/graphql/graphql-scalars

[sarahxsanders]

Thanks for reviewing and approving @saihaj! I don't have merge permissions here either :) so feel free to merge this whenever

[JoviDeCroock]

Same here, we need the word composability added to cspell.yaml

[sarahxsanders]

added! @JoviDeCroock