A big change for template paths landed in Hugo 0.146.0.
In `layouts`, we:
- remove `_default` and move everything in it directly under `layouts`
- rename `partials` and `shortcodes` to `_partials` and `_shortcodes`
- adapt to Hugo and docsy changes about the render-heading hook.
We don't need a copy of the heading self-link template now that it is
defined as a partial.
- update `docs/baseof.html` to match a change upstream
- split `docs/changelog.html` because it doesn't work for the section
page anymore. We create a `changelog-index` layout for this.
Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>
