Monitor lambda function that updates our links registry

[Mpdreamz]

We need monitoring to ensure we are alerted if the links registry fails to update.

This could cause PR builds to only see old links.

[reakaleek]

With #971 we are changing the architecture to use an SQS queue.

We can set up a dead letter queue for all unprocessed SQS messages and then somehow get alerted if the dead letter queue has messages or reaches a threshold.

[reakaleek]

Refs:

• https://github.com/elastic/docs-infra/pull/65

[reakaleek]

After adding https://github.com/elastic/docs-infra/pull/66, I was able to verify that the dead letter queue works as expected.