Add more information about levels for the grammar topics

[yurytsoy]

Currently most topics only have information about TTMIK. However, there are many other Korean learning resources, that can be referenced. Therefore, it would be good to add information about those resources as well.

Some candidates are (source):

• Korean grammar in use
• HTSK: https://www.howtostudykorean.com/
• Go! Billy: https://gobillykorean.com/

Additionally, at the moment the level is an integer number. This can be limiting, because sometimes the level can be a combination of letters and digits (eg A2) or a sub-chapter. Thus support more flexible ways to define a level would be useful.