add architecture design overview

[gavwu]

Describe your changes

1. fix gitignore for vim user
2. add a doc for current architecture design overview

Issue

• [ ] Check this box if this PR fixes an issue, and then paste the link below
• [x] No associated issue

Test:

• [ ] Check this box if this PR has tests
• [x] No test needed

[nascentcore-eng]

Thanks for the contribution!

I only have minor suggestion to the file name and location, otherwise all look good.