feat(mdx-loader): wrap mdx content title (# Title) in <header> for concistency
#6230
| Job | Run time |
|---|---|
| 3m 49s | |
| 3m 51s | |
| 3m 48s | |
| 4m 0s | |
| 3m 45s | |
| 3m 49s | |
| 3m 42s | |
| 3m 59s | |
| 3m 21s | |
| 34m 4s |