Docs: Promote perftools/php-profiler more earlier

[glensc]

As seems, people still want to install GUI aside from their application while there's is no such need to pull so many dependencies to their application.

Perhaps the problem is that the documentation is not clear enough and the projects used to be the same repository, but this is not the case for very long already.

refs:

• https://github.com/perftools/xhgui/issues/434#issuecomment-957172239
• https://github.com/perftools/xhgui/issues/434#issuecomment-1014746299
• https://github.com/perftools/xhgui/issues/278#issuecomment-1011439472

[glensc]

Doesn't look like anyone going to leave any feedback, so I'll go ahead and merge