chore(spectrum-two): config ci updates

[castastrophe]

Description

A few minor CI updates for linting:

• Remove *.mdx files from markdownlint as they don't adhere to the same guidelines
• Add the eslint config
• Only run markdownlint if markdown files are present

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

• I expect *.mdx files not to trigger the markdownlinter (@rise-erpelding)
• I expect eslint to use the local .eslintrc configuration
• I expect markdownlint to run only if *.md changes are present

To-do list

• I have read the contribution guidelines.
• ✨ This pull request is ready to merge. ✨

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Latest commit: c01f385

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PR #4163 has been deployed to Azure Blob Storage: https://spectrumcss.z13.web.core.windows.net/pr-4163/index.html.

[github-actions]

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Summary

Total size: 1.43 MB^*

🎉 No changes detected in any packages

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