* Include docs for Documentation Target API
* Rearrange the page to combine it with the deprecated Documentation Provider API
* Update API changes page
* Add notes to the language tutorial
* Include suggestions from Karol and Yann
* add docs about navigation bar for Custom Language Support topic
* add info in `topics/intro/content_updates.md`
* renumber articles after the added tutorial
* add `custom_language_tutorial_header` to `structure_aware_navbar.md`
* Initial version of a simple DocumentationProvider
* Correct usage of DocumentationMarkup, fix quick navigation
* Include Yann's suggestions. Not finished yet.
* Fix issues Yann pointed out