docs(events): add examples for RuleTargetInput and EventField

[krantboy]

Issue # (if applicable)

Closes #11210.

Reason for this change

RuleTargetInput already supports everything the issue asks for, but it was
undocumented. input.ts carried no examples at all, so the generated
RuleTargetInput API page
listed four factory methods with no indication of what to pass or what they produce.
The README did show RuleTargetInput in use, but only inside a CodeCommit walkthrough
and without a section of its own, so it read as a detail of that example rather than a
general capability, and it did not say which CloudFormation property each factory
method produces.

Description of changes

Documented on RuleTargetInput the three CloudFormation shapes it abstracts over
(Input, InputPath, InputTransformer) and when each is synthesized, and added an
@example to fromText(), fromMultilineText(), fromObject() and
fromEventPath(). The fromObject() example shows the method both with and without
EventField, since that is what decides between a literal Input and an
InputTransformer. Also documented on EventField how referenced paths become
InputPathsMap entries with <key> placeholders in the InputTemplate.

Added a short README section so the capability is discoverable from the module docs.
It notes that the prop accepting a RuleTargetInput is named per target (event on
LambdaFunction, message on SnsTopic) and that some targets such as EcsTask
shape their input differently.

Describe any new or updated permissions being added

None.

Description of how you validated changes

Documentation only, no runtime behavior affected. aws-cdk-lib builds and
run-rosetta.sh passes, so the new examples compile and translate to all target
languages. The synthesized values shown in the examples match the expectations in
aws-events/test/input.test.ts.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license