docs: fix content-type for *.md and *.txt files in docs deploy

[reidbarber]

Some unicode characters weren't getting rendered correctly (e.g. as â€) in *.md and *.txt files in the docs build. We need to be more explicit about the content-type here.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Before: https://react-aria.adobe.com/Calendar.md
Before: https://react-aria.adobe.com/llms.txt

After: https://d5iwopk28bdhl.cloudfront.net/pr/b11acecd045b658f4247ce3dfc019d7453350e84/Calendar.md
After: https://d5iwopk28bdhl.cloudfront.net/pr/b11acecd045b658f4247ce3dfc019d7453350e84/llms.txt

Cmd+F search to confirm that â€ isn't in the new output.

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs