Fix: Add custom focus outline for <summary> element

Fix: Add pointer cursor on <summary> element

Fix: Do not explode when on null in replace (styleguidist#1115)

Fix: Add styles to <details> and <summary> elements in Markdown (styl…

…eguidist#1099)

Closes styleguidist#991

Changelog: 🚀

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## New features

* Added support for external links in sections ([styleguidist#921](styleguidist#921) by @BTMPL)

Two flags added to `Sections` config:

* `href` - An URL to navigate to instead of navigating to the section content
* `external` - If set, the link will open in a new window

Fix: Support at-return as a synonym of at-returns

Simplify code a bit.

Fix: Appear page not found when a component is isolated (styleguidist…

…#1091)

Fix: Make slot components overridable (styleguidist#1087)

Fixes styleguidist#1070

Fix: Fix preview of method optional argument, JSDoc param and return …

…synonyms (styleguidist#1046)

Read 'param', 'arg', 'argument' JSDoc tags for method params, and 'return', 'returns' for method return value.

Fix: Fix section name regression

Fixes styleguidist#1078