doc: Improve SW architecture diagram

[elsalahy]

Summary

Minor improvement to the SW architecture diagram after some internal reviews. Relates to #40

Screenshots

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Changes

• Update the application layer of the SW architecture diagram.

Notes for Reviewers

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Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• [x] New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• [x] Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• [x] Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.