[doc] Rewrite most frontmatters to Markdown titles

Completely throw away the frontmatter and only keep the title which is
turned into a Markdown title.

To recreate the rewrite, run the added script,
`util/rewrite_frontmatter.py`, in the root of the repo.

```sh
python3 util/rewrite_frontmatter.py ./
```

Signed-off-by: Hugo McNally <[email protected]>
Co-authored-by: Amaury Pouly <[email protected]>

Author: 2 people