docs(nav): make REST and GraphQL event handlers more explicit

[heitorlessa]

Issue #, if available: #857

Description of changes:

Changes Event Handler ApiGateway title to REST API, and AppSync to GraphQL API while keeping URL the same. Enables navigation indexes feature and move tutorial to tutorial/index.md to keep navigation while allowing for growth - additional tutorials as customers request.

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

Breaking change checklist

RFC issue #:

• Migration process documented
• Implement warnings (if it can live side by side)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

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View rendered docs/core/event_handler/api_gateway.md
View rendered docs/core/event_handler/appsync.md
View rendered docs/tutorial/idempotency.md
View rendered docs/tutorial/index.md

[codecov-commenter]

Codecov Report

Merging #959 (537a0a0) into develop (605e9fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #959   +/-   ##
========================================
  Coverage    99.96%   99.96%           
========================================
  Files          119      119           
  Lines         5320     5320           
  Branches       613      613           
========================================
  Hits          5318     5318           
  Partials         2        2           

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