Add versioning header to help translators keep V by Example up-to-date

[vbrazo]

The idea is to implement a header to control the version of each lesson in this project.

Here is an example:

---
version: 3.5.1
example_title: JSON
---

You should change the version attribute according to the following rules:

MAJOR — You (re)wrote the whole thing. Your new content will need some translation.
MINOR — Added or removed some content, few sentences, etc.
PATCH — Spelling, typos. Probably not translated stuff.

We follow SemVer.org pattern. This is important because we use that to determine and inform translators of new content that requires translation.