[events] move `write-started` from `Builder.write` to `Builder.build` method

[chrisjsewell]

This event was recently added in https://github.com/sphinx-doc/sphinx/pull/12567, but as pointed out in #12680 this may not fire for all builders, since write can be overriden.

I can't recall if there was any additional thinking as to its original placement, but certainly with https://github.com/sphinx-doc/sphinx/pull/12436, we can "guarantee" that it will be called for all builders when called in build.