DOCS: Replacing architecture diagrams

[suearkinunity]

Description

manual/Architecture.html has two old diagrams. They're very hard to read, and we've had user complaints about that.

This is an attempt to replace them with Mermaid diagrams.

Testing status & QA

Docs only

Overall Product Risks

None: docs only

Comments to reviewers

Very different way to construct the diagrams, so requires careful review.

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • JIRA ticket linked, example (case %%). If it is a private issue, just add the case ID without a link.
  • Jira port for the next release set as "Resolved".
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[cla-assistant-unity]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@             Coverage Diff             @@
##           develop    #2448      +/-   ##
===========================================
- Coverage    58.58%   58.38%   -0.20%     
===========================================
  Files          738      738              
  Lines       135831   136439     +608     
===========================================
+ Hits         79570    79658      +88     
- Misses       56261    56781     +520     

Flag	Coverage Δ
inputsystem_Windows_6000.0	5.31% <ø> (-0.01%)	⬇️
inputsystem_Windows_6000.3	5.31% <ø> (-0.01%)	⬇️
inputsystem_Windows_6000.6	5.30% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 6 files with indirect coverage changes

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