Rework Eventing information architecture

Rework the existing Eventing IA.

Draft PR link: https://github.com/couchbase/docs-server/pull/3453

Current IA:

• Eventing Service: Fundamentals
  • Terminology
  • Language Constructs
    • Advanced Keyspace Accessors
    • Timers
    • cURL
  • The Eventing Lifecycle
  • Buckets vs Collections
  • Eventing Role-Based Access Control
  • Debugging and Diagnosability
  • Statistics
  • Troubleshooting and Best Practices
  • Frequently Asked Questions

Possible new IA:

• Introduction to Eventing
  • Create an Eventing Function
  • Use the Eventing Service
  • Custom Eventing Functions
  • Eventing Scriptlets
  • Debug Eventing Functions
  • Fetch Eventing Statistics
  • Eventing Concepts
  • Troubleshooting and FAQs

More detailed possible new IA:

• Introduction to Eventing
  • Create an Eventing Function
  • Use the Eventing Service
    • Data Enrichment (needs better title)
    • Perform a Cascade Delete Operation
    • Create an Expiration Function
    • Differentiate Between Deletion and Expiration
    • Create an Archiving Function
    • Create a Recurring Timer
    • Cancel or Overwrite a Timer
    • Fetch Daily Exchange Rate Data
    • Perform Risk Assessment
• Custom Eventing Functions
• • Basic Keyspace Accessors
  • Advanced Keyspace Accessors
  • Logging
  • SQL++ Statements
  • N1QL() Function
  • crc64() Function
  • Timers
  • cURL
  • Added Language Features
  • Removed Language Features
  • Built-in Functions
  • Handler Signatures
  • Reserved Words
• Eventing Scriptlets
  • Basic KV Functions
  • Basic SQL++ Functions (maybe combine with "SQL++ Statements" page above?)
  • Generic Manipulation Functions
  • Advanced Accessor Eventing Functions (maybe combine with "Advanced Keyspace Accessors" page above?)
  • Binary Document Support
  • Performance Functions
• Debug Eventing Functions
• Fetch Eventing Statistics
• Eventing Concepts
  • Buckets vs Collections
  • Role-Based Access Control
  • Terminology
• Troubleshooting and FAQs