Refactor docs

Fix #94.

Add bundle activation to installation

Improve @Tag section in Annotations

Include match partial in invalidation chapter

Update user context docs

Fix typos

Add chapter intros

Add chapter headers

Add Debug config chapter

Add flash message chapter (todo)

Update proxy client chapter

Remove default config chapter

Add Configuration chapter header

Re-organise configuration chapters

Match is now linked to instead of included, and included in the TOC too.
various tweaks and cleanups
Finalise docs

* fix links to FOSHttpCache docs
* a little clean-up
Merge pull request #111 from FriendsOfSymfony/refactor-docs

Refactor docs