docs: revamp simplified architecture diagram
What does this MR do and why?
Revamps simplified architecture diagram, initially introduced in gitlab-foss!28729 (merged), as follows:
Technical
- Remove connection between GitLab Shell and Redis from updated GitLab basic architecture diagrams (14.8) (#354646 (closed))
- Indicate GitLab Shell talks directly talks to Workhorse (instead of Puma) (13.4+) (#354881 (closed))
- Remove the line between Shell and Puma from the simplified diagram
- Add a line from Shell to Workhorse in the simplified diagram
 
Design
- Lines are more straight as possible.
- Layers are used to align each component.
- Image size is changed from 1410x980 to 1022x1444.
- File size increases slightly from 27KB to 43KB.
- Visual GitLab logo is removed.
- Purple color is replaced with black/white tone.
Screenshots or screen recordings
The updated image says all. See changes.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
- 
I have evaluated the MR acceptance checklist for this MR. 
Closes #354646 (closed) and #354881 (closed)
Closes #229893 (closed)
Edited  by Takuya Noguchi