Developer docs: replace diagram with table for inter-service communication

[backmari]

Description of the changes

Check all that apply:

• updated documentation
• Source added/refactored
• Added unit tests
• Added integration tests
• (If applicable) Verified that manual tests requiring the /SNS and /HFIR filesystems pass without fail

References:

• Links to IBM EWM items:
• Links to related issues or pull requests:

Manual test for the reviewer

(Instructions for testing here)

Check list for the reviewer

• best software practices
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

[petrasrjornl]

Reviewed the resulting doc in red-the-docs, the updated section looks good for content.