Clarify the term "modules"

[patiencedaur]

Epic: Restructuring Tarantool docs

Details

@ NeraverinTarantool's viewpoint

The term "module" has many different meanings:

• a Lua library that simplifies writing Lua code
• an application like Cartridge CLI
• a framework like vshard or Cartridge that influences the whole architecture

Besides, modules have different installation methods:

• a built-in module
• an external Lua rock
• a community module
• a standard Lua rock

@ kyukhin's viewpoint

• Cartridge CLI, just like tarantoolctl, is not a module but an application and should be called so
• Modules can be classified in several ways:
  • public/closed-source
  • standard Lua rocks/created by the Tarantool team/community modules
  • built-in/external
• Large entities like vshard and cartridge might need to be called something other than "modules".

TBD