[WIP] Documenting the Validator component

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | 2.3+
| Fixed tickets | #958

I started documenting one of the last missing components (only
BrowserKit needs some love after this).

This might become just another refactoring of the documentation, since
it'll duplicate a lot of `book/validation`. I don't think it's a problem
if we do the same thing as we did with the translation documentation?
(that was a test, but I think it has gone well?)

Todo
---

- [ ] Moving the definition part from the book chapter
- [ ] Writing about validating values (including how to deal with
  violations, like translation, presentation, debugging, etc.)
- [ ] Fix the book chapter

Todo after this PR is merged (will be transformed into issues)
---

- [2.5] Update the caching stuff to https://github.com/symfony/symfony/pull/9892
- [2.5] Update for https://github.com/symfony/symfony/pull/10287

/cc @ricardclau, @webmozart