Create In-app User Guide

[c0deRae]

Creating an in-app user guide for basic and advanced features of YACReader.

• Gather most frequently asked questions from yacreader forum, gh issues and discord channels.
• Organize that data in a manner that makes sense such as how a user would logically approach using YACReader. I.e opening a comic, creating a library, using the server, etc.
• Write guide in Markdown as it is supported by Qt 5.15+ and would be displayed in GitHub and in-app.