feedback: Troubleshooting guide

> <…>eeds thorough
> code analysis aimed at optimizing memory usage.
> 
> 
> **|Problem: Fiber switch is forbidden in __gc metamethod|**
> 
> Problem description
> Fiber switch is forbidden in __gc metameth<…>

https://www.tarantool.io/en/doc/latest/book/admin/troubleshoot/#problem-fiber-switch-is-forbidden-in-gc-metamethod

only 'metamethod' is visible in the table of contents on the right