As we gain multiple guides (e.g. Developers Guide, User Guide, API
Guide), the /index.html should be reserved as a landing page.

Instead of using /index.html while the Solidus Guides are only the
Solidus Developers Guide, let's redirect the homepage to
/developers/index.html.

This commit adds helper methods that will make rendering different
tables of contents on different parts of the site easy.

This also updates the discover_title helper to account for multiple
tables of contents.

\## Dealing with multiple tables of contents

For example, if I visit the following URL:

`localhost:4567/developers/orders/overview.html`

I should see the table of contents for the "Solidus Developers Guide".
And if I visit this URL:

`localhost:4567/users/promotions/overview.html`

I should see the table of contents for the "Solidus Users Guide".

Here's a description of the helpers's functionality:

\## table_of_contents

The table_of_contents helper allows you to determine which table of
contents should be rendered. If the user is viewing a URL with the
subdirectory `/developers/`, then the current table of contents will be
at the `/data/nav/developers.yml` YAML file.

\## current_guide

the current_guide helper finds which guide the user is viewing. If they
are a URL with the subdirectory `/developers/`, then the current guide
is the "Solidus Developers Guide".

The current_guide returns the YAML at `/data/nav/global.yml` which
allows you to get the `current_guide.title` and `current_guide.href` for
the current guide.

This commit enables multiple table of contents rendering for the Solidus
Guides.

Each table of contents can have as many "submenus" as you want. Here is
the YAML structure for a short submenu:

```
-
  - title: "Section name"
    dropdown:
      - title: "Article name"
        href: "/relative/link/to/article.html"
      - title: "Another article name"
        href: "/relative/link/to/another-article.html"
```

Things you should note about the structure:

- Each submenu is represented by a root `-` node that is unnamed.
- Each submenu can have multiple sections and multiple articles.
- Articles need to be included in a `dropdown:` at this point in time.

\## Global and Meta tables of contents

The `/data/nav/global.yml` and `/data/nav/meta.yml` do not follow this
data structure.  These are special menus that will be used throughout
the site and should not be rendered only when the user is on specific
pages.

Each table of contents provides a link to the `current_guide.href`. In
the case of the "Solidus Developers Guide" table of contents, that links
to `/developers/index.html`.

These links should be styled to look like the <h1> of the table of
contents block. This is similar to how the title of a blog is usually
the color of a heading or the site's logo, not the color of a link.

It allows to access page as users/page.html instead of users/page
like we do into other sections.

This was raising an error in the middleman console

by renaming product-stock.md into product-stock.html.md

This is needed to switch between Developers and Users guides. This
commit also changes the header structure a bit to accomodate this
new navigation.

It also please Rubocop by removing Ruby 2.3 syntax which makes
Hound to complain.