Document sub-modules of the `polkadot_sdk` module

[juangirini]

Document all sub-modules of the polkadot_sdk module. Gather feedback from experts where needed, such that we know we have a good explanation of each component of the system.

Tasks

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1. Frame Runtime
2. Cumulus
3. Substrate
4. XCM documentation #58
   L1-ref-docs +1
   
5. Smart Contracts documentation #56
   L1-ref-docs +1
   
6. Finish up Templates document #67
   +0
   
7. Finish up Polkadot document #66
   +0