docs: mention document files for mdn/content can be in .md (mdn#5684)

* Mention document files for mdn/content can be in .md
* Mention index.md is recommended

Co-authored-by: Jean-Yves Perrier <[email protected]>
Co-authored-by: Schalk Neethling <[email protected]>

fix:chore ~ ensure consistency in usage of extraClasses (mdn#5839)

This ensures we consistently use the same pattern when using `extraClasses`.
It also updates some of the typings to always set `extraClasses` to options and
the type to either `string` or `null`.

Also removes the unused Select component.

Co-authored-by: Claas Augner <[email protected]>

Inline notecard: reduced its size (mdn#5846)

fix:plus ~ update to product overview (mdn#5841)

Merge pull request mdn#5832 from mdn/88-no-empty-state-message-in-col…

…lections

fix:collections ~ add copy to empty state

Merge pull request mdn#5826 from caugner/build-spas-for-plus-docs

fix: build SPAs for Plus docs + allow indexing Plus product page

fix: Dark mode weird white spot with both scrollbars visible (mdn#5842)

build(deps-dev): bump node-dev from 7.3.1 to 7.4.1 (mdn#5838)

build(deps-dev): bump babel-loader from 8.1.0 to 8.2.4 (mdn#5739)

Make it possible to use an absolute URL when providing a redirect (md…

…n#5814)