Update Theme

[thiagooak]

The haiku theme looks a bit dated. How about updating to the sphinx_rtd_theme theme?

[jpauli]

Ah yes, I like this one and can see it in several places 👍

[nikic]

Generally seems like a good idea, but there's probably going to be some issues -- e.g. the sidebar in your screenshot doesn't separate PHP 5 and PHP 7 in any way, so it ends up having two introductions etc. Probably need to make those explicit in the TOC?

[thiagooak]

Yeah. We would need to tweak to TOC