[Book][Internals] Add "kernel.terminate" event to Internals section

[tPl0ch]

The kernel.terminate event is listed in http://symfony.com/doc/current/components/http_kernel/introduction.html#the-kernel-terminate-event, but should IMO also be listed here: http://symfony.com/doc/current/book/internals.html#events

Unfortunately I don't have the time right now to fix it, but I open this issue as a reference so somebody else can pick it up.

[wouterj]

The internals article is a little outdated. In fact, it is not changed since it was written in 60dd8a4 (2 years ago).

This is one of the articles that should get on top of the 're-reading' queue.

I've labeled it with Easy pick because someone just needs to read the section in the http_kernel/intro article and he'll be ready to write this.

[stof]

@wouterj this article should probably be removed altogether (or at least large parts of it) as components are now documented on their own.

[wouterj]

@stof that's maybe an option too.

/cc @weaverryan @fabpot

[tPl0ch]

@wouterj @stof Probably the section could be used as a comprehensive list of important core features (possibly with short introductions) with links to the relevant component's documentation.

[cordoval]

#2405