refactor: documentation structure

[kinten108101]

/docs should only contain relevant documentations i.e. the workings of SteamVpk.

So far, all three doc files have been about GTK usage, which is personal. In the source code, there are comments about GTK usage. Given this situation, some files have to be refactored.

The documentation locations in question are:

• /docs
• Comments

Acceptance criteria

• All documentation content are moved to the right place

Commentary

I also took the time to improve some docs, specifically Requirement Analysis.md. Now it is Overview.md and contains important information about the projects: the architecture, coding style, etc. This file, however, is only temporary. For example, the coding style section is just a reference to a Youtube video. In the future we will have better means of communicating styles, and it will have a separate file (probably through a contributor/hacking guide).