[BUGFIX] Correct Menu levels in global menu tree

[linawolf]

Resolves #803

@wouterj does this resolve your issue?

[wouterj]

Thanks for trying to tackle the issue.

This fixes the case for level 1, but it seems like all sub nodes are still one up:

<div class="menu-wrapper menu-size-md">
    <ul>
        <li class="toctree-l1">
            <a href="/getting_started/index.html#getting-started">Getting Started</a>
            <ul>
                <li class="toctree-l3">
                    <a href="/setup.html#installing-setting-up-the-symfony-framework">Installing &amp; Setting up the Symfony Framework</a>
                    <ul>
                        <li class="toctree-l4">
                            <a href="/setup/bundles.html#upgrading-a-third-party-bundle-for-a-major-symfony-version">Upgrading a Third-Party Bundle for a Major Symfony Version</a>
                        </li>
                        <li class="toctree-l4">
                            <a href="/setup/file_permissions.html#setting-up-or-fixing-file-permissions">Setting up or Fixing File Permissions</a>
                        </li>
                        <!-- ... -->

[linawolf]

Thanks @wouterj, I will add tests for that

[linawolf]

@wouterj please have another look