docs: enhance README with technical architecture details

[minhyeong112]

Resolves ubiquity-os-marketplace/.github#4

[0x4007]

I fear this may immediately be out of date because I know we have a large pending pull request.

@whilefoo perhaps you can use cline the vscode extension to regenerate a current readme after merge.

[gentlementlegen]

I really think that paraphrasing the code inside of the docs is very bad practice, and quite useless because it becomes obsolete whenever the code is changed. Linking to the files would be more interesting, instead of copy pasting content (like for the manifest). Also keep in mind these docs appear side by side within our plugin installer, which would show a lot of useless information.

As a dev when I read docs, I expect a quick setup example, an overview of capabilities and how to configure, not what code files do.