[Docs] Changes to navbar documentation/explanation (#32090)
[Docs] Changes to navbar documentation/explanation (#32090)

* Expand "external content" section for navbar

- rephrase the intro to make a bit more sense
- add note about dangers of focus/reading order problems

* Remove pointless intro sentence

* Small wording changes/additions/fixes
tjkohli/bootstrap,GerHobbelt/bootstrap,m5o/bootstrap,GerHobbelt/bootstrap,coliff/bootstrap,tjkohli/bootstrap,coliff/bootstrap,twbs/bootstrap,inway/bootstrap,GerHobbelt/bootstrap,inway/bootstrap,m5o/bootstrap,twbs/bootstrap,inway/bootstrap