Add extension list

[szkiba]

It is usual for awesome lists that although the list is longer, the list directly contains links to resources. In addition to the GitHub topic search (xk6), it is advisable to add a list of popular extensions.

Extensions supported by Grafana and supported by the community should be placed in separate sections. The list of community-supported extensions should initially contain those registered extensions whose repository has ten or more stars. Later, this list will be maintained by the community (in the form of PRs).