Merge pull request #892 from just-the-docs/v0.4.0-dev

.devcontainer/devcontainer.json

.stylelintrc.json

.vscode/tasks.json

404.html

CHANGELOG.md

_config.yml

_includes/css/callouts.scss.liquid

_includes/css/just-the-docs.scss.liquid

_includes/favicon.html

_includes/head.html

_includes/icons/external_link.html

_includes/mermaid_config.js

_includes/nav.html

_includes/nav_footer_custom.html

_includes/search_placeholder_custom.html

_layouts/default.html

_sass/base.scss

_sass/buttons.scss

_sass/code.scss

_sass/color_schemes/dark.scss