docs: add example with custom tick label strings

[joelostblom]

This adds an example of how to replace numerical tick labels with custom strings using a nested ternary expression inside labelExpr. Let me know if I missed something for adding an example, I wasn't sure about adding the <span> tagged line to the site docs as mentioned in the contributor guidelines since other examples did not seems to do this.

close #7045

• Make the pull requests (PRs) atomic (fix one issue at a time). Multiple relevant issues that must be fixed together? Make atomic commits so we can easily review each issue.
• Provide a concise title as a semantic commit message (e.g. "fix: correctly handle undefined properties") so we can easily copy it to the release note.
  • Use imperative mood and present tense.
• Mention relevant issues in the description (e.g., Fixes #1 / Fixes part of #1).
• Lint and test (Run yarn test).
• Rebase onto the latest master branch.
• Review your changes before sending the PR (to ensure code quality).
• For new features:
  • Add new unit tests.
  • Update the documentation under site/docs/ + add examples.

[domoritz]

Replaced by #7049 so we can build examples on the ci.