This commit is the first approach to address typelevel#1678, developing the
form-validation example.

This commit contains a simple form-validation example and a couple
of approaches making use of `Validated`.
It aims to solve typelevel#1678

- The code examples for form validation are now using `tut` with
its respective output in the generated .html.
- Fixes some structural issues (`Either` return type in the `Validated`
example, use of `mapN` instead of `|@|`).
- Adds a deprecation notice about cartesian builder and changes this
for using `.mapN`.

- The `Validated` approach with the for-comprehension has been
splitted in two: the instructions that compile and the for-comprehension
that doesn't compile. For the first ones, I've used `tut:silent` and
for the second one I've used `tut:book:fail`. With this, all the code
of this proposal is checked by tut correctly.
- Adds an import for Either to bring .flatMap in Scala versions prior
to 2.12.x.

- Fixes indentation in all the snippets (two spaces).
- Added a new section ("short detour") in where I explain the
`Semigroup` role in the accumulation, with a little portion of
`Validated` code.
- Adds "Back and forth" section explaining how to convert between
`Validated` and `Either`. I can't figure it out how to convert from
`Either` to `Validated` 'failing slowly'.

- Added `final` modifier to `RegistrationData` case class.
- Changed if-else in `Either` example to `Either.cond`.
- Reused `Either` example validations in `Validated` (non-compiling)
approach with `.toValidated`.
- Deleted redundant imports for tut.
- Changed `Validated` of a `NonEmptyList` for `ValidatedNel` type alias.
- Deleted 'disclaimer' about converting from `Validated` to `Either`,
losing the fail-slow functionality.
- Added `Semigroup` example based on the Peter's one but in the
context of the example provided here.
- Fixed some typos and grammar errors.
- Pointed out to the cats `Semigroup` documentation instead of Wikipedia.