Fix doc build instructions

It is necessary to do a `deps.get` before calling `compile` or we get
this error:

    Unchecked dependencies for environment dev:
    * earmark (Hex package)
      the dependency is not available, run `mix deps.get`
    ** (Mix) Can't continue due to errors on dependencies
Improve Mix doc intro section

- fix a few typos
- add `use Mix.Project` in examples or we get this error:
    ** (Mix) Could not find a Mix.Project, please ensure a mix.exs file
    is available
- use real version of ecto in example, or we get this error:
    ** (Mix) No package version in registry matches ecto ~> 0.3.0
Merge pull request #2937 from amarandon/doc-fixes

Some documentation corrections