Improve docs coverage with sclicheck

[Gedochao]

This improves the coverage of code included in our docs with the following

• markdown code blocks are now handled on par with scala and java (so that markdown examples can be compiled and tested)
• code blocks with >3 backticks are now supported (which also enables nesting code blocks in markdown examples)
• some docs have been skipped until now, as they didn't include md or bash code blocks, but they still did have stuff to be compiled and tested (i.e. using-directives.md)
• the docs root folder is now included in sclicheck.DocTests

▶ ./mill -i docs-tests.test 'sclicheck.DocTests.root*'
(...)
sclicheck.DocTests:
  + root getting_started 18.253s
  + root release_notes 0.007s

• the reference docs folder is now included in sclicheck.DocTests

▶ ./mill -i docs-tests.test 'sclicheck.DocTests.reference*'
(...)
sclicheck.DocTests:
  + reference scala-command/index 0.129s
sclicheck.SclicheckTests:
sclicheck.GifTests: