Add GraphQL input types to new GraphQL docs

What does this MR do and why?

This MR adds GraphQL input types to the new GraphQL docs.

It is part of a series of MRs to move across the work in !130378 (closed) in smaller pieces.

Previous MRs:

MR	Description
!243355 (merged)	Enums
!250737 (merged)	Scalars

References

Split GraphQL API reference documentation into ... (#593121)

How to set up and validate locally

You can review the generated docs for this MR:

• Index https://docs.gitlab.com/upstream-review-mr-ee-251517/api/graphql/reference/experimental/
• Input types page https://docs.gitlab.com/upstream-review-mr-ee-251517/api/graphql/reference/experimental/input_objects/

Try changing a GraphQL input type locally, for example:

diff --git a/app/graphql/types/ci/variable_input_type.rb b/app/graphql/types/ci/variable_input_type.rb
index ..  100644
--- a/app/graphql/types/ci/variable_input_type.rb
+++ b/app/graphql/types/ci/variable_input_type.rb
@@ -4,7 +4,7 @@ module Types
     class VariableInputType < BaseInputObject
       graphql_name 'CiVariableInput'
-      description 'Attributes for defining a CI/CD variable.'
+      description 'Attributes for defining a CI/CD variable. Test.'

       argument :key, GraphQL::Types::String, description: 'Name of the variable.'

Then run the check docs rake task:

bundle exec rake gitlab:graphql:docs:check

It should detect that there are changes that haven't been committed to the compiled docs:

##########
#
# Experimental GraphQL documentation is outdated!
# Please update the documentation by running `bundle exec rake gitlab:graphql:docs:compile`.
#
##########

Now compile the docs:

bundle exec rake gitlab:graphql:docs:compile

It should compile successfully:

Experimental documentation compiled.

And run :check again, it should pass this time:

Experimental GraphQL documentation is up to date.

Check that the change to doc/api/graphql/reference/experimental/input_objects.md looks correct:

(Note you'll see changes to enums.md - that's fine - ignore those here, just look at input_objects.md)

git diff doc/api/graphql/reference/experimental/input_objects.md

The diff should be:

diff --git a/doc/api/graphql/reference/experimental/input_objects.md b/doc/api/graphql/reference/experimental/input_objects.md
--- a/doc/api/graphql/reference/experimental/input_objects.md
+++ b/doc/api/graphql/reference/experimental/input_objects.md
@@ ## `CiVariableInput`

-Attributes for defining a CI/CD variable.
+Attributes for defining a CI/CD variable. Test.

MR acceptance checklist

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Related to #593121