Sort modules by purpose in the reference

[patiencedaur]

In our https://www.tarantool.io/en/doc/latest/reference/, modules are sorted alphabetically, which is a bit messy. Consider the header "Module say (logging)", which is different from the others because it explains the purpose of the modules.

In Awesome Tarantool, modules are sorted by their purpose. Users make sense of this sort more easily.

Tentative plan:

• Add umbrella topics for modules in the reference
• Add links to missing module docs. Initially they can be links to GitHub wiki, external pages, etc.