Add article "Customize performance profile with extensibility APIs" (#3406)

* draft

* update detailed toc

* whitespace

* linkfix

* test backticks in link text

* backticks in headings

* fresh re-paste, some pngs

* See Also links

* ***

* syntax, Gallery code

* Customize mark()

* linkfix

* link timeStamp() Ref

* Incorp review cmts

* Update code and pngs

* Update Detailed Contents

* incorp review pngs

* prop Show custom tracks steps

* remove dup steps

* remove cmt

* Photo selected custom marker

* event

Author: mikehoffms