docs: adds examples of typed custom field components (#10741)

Although the "customizing fields" doc provides a big picture overview of
how to create custom field components, it is not explicit enough for
developers to know exactly where to start. For example, it can be
challenging to import the correct types when building these components,
and the natural place to go looking for this information is on the
fields docs themselves. Now, each field doc has its own dedicated
"custom components" section which provides concrete examples for fields
and field labels in both server and client component format, with more
examples to come over time such as using inputs directly, etc. In the
same vein, the "customizing fields" doc itself should probably be moved
to the fields overview section so it remains as intuitive as possible
when searching for this information.