Text updates: aws/aws-lambda-monitoring

Includes minor text revision in README, as well as these documented in the changelog:

Revise text labels and prompts in configuration modal
Remove "none" option for providing credentials for consistency among integrations
Add summary for success screen in integration yaml file
Add several setup steps and options in README
Update README link references to match headings on the linked pages
Add alert for aws_lambda_dead_letter_errors in README
Reformat metrics table in README
Add reference documentation list

sensu / catalog