Invoke plantuml jar from the respective working directory

[tlylt]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Fixes #1752

Style file inclusion was not working properly as we invoke the plantuml jar file at the root directory, hence the file path resolution cannot find the correct style.puml. This fix configures the call the plantuml executable to use the output folder that contains the .puml to be the current working directory. This helps with relative path resolution when a .puml includes other .puml files.

Anything you'd like to highlight/discuss:
nil

Testing instructions:

• Clone https://github.com/tlylt/ab3-docs-demo
• cd docs
• With local master markbind, run markbind serve
• visit /DeveloperGuide.html
• manually refresh
• observe that the Architecture diagram is broken
• switch to this branch, rmb to run npm run build:backend
• then rerun markbind serve
• may need to manually refresh
• check that the diagram is working
  

Proposed commit message: (wrap lines at 72 characters)
Invoke plantuml jar from the respective working directory

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[ong6]

LGTM