[1/?] Add ability to run code examples in the playground: Move code samples to separate files

[shaedrich]

See #340

Fun fact: Loading snippets from gists, would have been possible via url_download config option 😉

Key points

• top-level /code-samples directory
• flat structure within /code-samples
• file names within /code-samples having the page name, they are primarily used in as a prefix
  • a few files only consist of said prefix
• file names consist of [a-z-]
• code samples are embedded, using the snippet feature with the two dashes each syntax (--8<--)
• code samples are reused when the same code is used elsewhere or lines from it (snippet lines syntax). Code sample reuse, that could only be achieved by the block format is not done
• code samples currently have no new line at EOF
• we currently have code samples in the following languages:
  • pony (see below)
  • Bash (.sh; 13)
  • C (9)
  • Error (.txt; 3)

Progress

• [x] Getting Started (16 code samples, 12 unique snippets)
• [x] Types (46 code samples, 43 unique snippets)
• [x] Expressions (131 code samples, 129 unique snippets)
• [x] Reference Capabilities (22 code samples, 21 unique snippets)
• [x] Object Capabilities (6 code samples, 4 unique snippets)
• [x] Generics (22 code samples, 18 unique snippets)
• [x] Packages (8 code samples/snippets)
• [x] Testing (6 code samples/snippets)
• [x] C-FFI (34 code samples/snippets)
• [x] Gotchas (8 code samples/snippets)
• [ ] Where Next?
• [x] Appendices (36 code samples/snippets)

= 313 code samples, 298 unique snippets

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