Add program flow page to developer guide

[ang-zeyu]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [x] Documentation update

Resolves #812
Part of #1027

What is the rationale for this request?

• introduce the architecture of markbind and key processes in generating sites

What changes did you make? (Give an overview)

• Add program flow page to developer guide with two activity diagrams illustrating the key processes
• remove outdated / redundant info in devGuide.md

Proposed commit message: (wrap lines at 72 characters)
Add program flow to developer guide

[ang-zeyu]

ready for review https://deploy-preview-1224--markbind-master.netlify.app/devguide/design/architecture

any thoughts? :0
@ryoarmanda @wxwxwxwx9

[wxwxwxwx9]

@ang-zeyu sorry for the late response! I have taken a look at it and compared it with the codebase. It seems pretty accurate and clear. I think it's good! At first, I was wondering if we should also include PluginManager but perhaps it isn't that relevant to the diagram, right? Since it isn't involved in the content processing flow.

[ang-zeyu]

thanks for the comments! @wxwxwxwx9

I've made some changes to the diagram and added some additional snippets of info elsewhere for the points you raised:

• not too clear that content processing flow is for a single source file - added a popover
• <panel>s (Externals) are separate from the content processing flow - added some tip boxes

let me know if there's anything else that can be improved here 🙂

[ryoarmanda]

Just checked the docs and the diagram looks nice! It shows a clear organization of the classes and flows, and the writeup is good for a high-level overview. Looks quite complete as well, so far most of the things I noticed first time through the codebase have been addressed 👍

[ang-zeyu]

Merged. Thanks again for the review : ) @wxwxwxwx9 @ryoarmanda