docs: Move code snippets to external files

[matevz]

Currently, all example code is hardcoded inside .md files. If we regulary change oasis-sdk API, the example code in docs may not work anymore. We should move the relevant code snippets to standalone .go and .rs files and CI should build it:

• Move rust and go snippets in the following markdown files:
  • https://github.com/oasisprotocol/oasis-sdk/blob/main/docs/contract/hello-world.md
  • https://github.com/oasisprotocol/oasis-sdk/blob/main/docs/contract/confidential-smart-contract.md
  • https://github.com/oasisprotocol/oasis-sdk/blob/main/docs/runtime/minimal-runtime.md
    to examples/contracts and examples/runtimes accordingly.
• Add tests to oasis-sdk CI to build the examples
• Wait for code snippets remark plugin to be implemented: Code snippets Remark plugin docs#225.