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
No need for default `code` arg in `IO.ANSI.Sequence.defsequence/3`
Simplify color sequences definition in `IO.ANSI`
Merge pull request #2940 from lexmag/ansi-colors

Simplify color sequences definition in `IO.ANSI`
Merge pull request #2939 from lexmag/master

No need for default `code` arg in `IO.ANSI.Sequence.defsequence/3`
Use same verb tense across Elixir documentation

The norm is to use passive present tense ("Returns bacon") and not
imperative ("Return bacon").
Remove dead @doc snippet
Merge pull request #2943 from lstor/remove-dead-doc

Remove dead @doc snippet
Merge pull request #2942 from lstor/doc-verb-tense

Use same verb tense across Elixir documentation
Add support for --werl in Windows bash-like shells
Merge pull request #2944 from lstor/elixir-werl

Add support for --werl in Windows bash-like shells
load modules from load path
Merge pull request #2932 from tonini/add-modules-load-path-to-autocomplete

Fetch module names from load path for autocomplete
Always use the loaded application modules
Merge branch 'master' of https://github.com/elixir-lang/elixir