preserve tabs in codefences (#18638)
Fixes #18622
Summary of the issue:
Tabs are replaced with spaces in codefences when converting markdown to
html.
This causes examples for characterDescriptions, gestures and symbol
files to contain incorrect syntax.
Description of user facing changes:
Fixed example in dev guide
Description of developer facing changes:
- Updated markdown
- added [python markdown extensions
suite](https://github.com/facelessuser/pymdown-extensions)
- deprecated DEFAULT_EXTENSIONS
Description of development approach:
- enabled preserve tabs
(<https://facelessuser.github.io/pymdown-extensions/extensions/superfences/#preserve-tabs>)
- made markdown extensions ordered
Testing strategy:
Built dev guide, ensured tabs are preserved
Built developerGuide and user_docs with and without this PR, and diffed the files to ensure only correct changes were made.
Known issues with pull request:
None
