docs/app serves `.md` without an encoding

[masenf]

Describe the bug
markdown files are served with default encoding, which has broken characters on my machine.

To Reproduce

cd docs/app
uv run reflex run

http://localhost:3000/docs/getting-started/introduction.md

Expected behavior
There shouldn't be broken characters.

Screenshots

[linear]

ENG-9444

[Alek99]

This issue needs to be fixed with someheaders in our cloudflare

[carlosabadia]

They seems to be right in the deployed app https://reflex.dev/docs/getting-started/installation.md

locally, looks like Vite serves .md as text/plain and never appends a charset