Use the correct internal link format in Rebel Engine API

[madmiraal]

Currently, when building Rebel Documentation, links from Rebel Engine API to other pages in Rebel Documentation are not tested, because they are created as external links. This has allowed various broken internal Rebel Documentation links (see #89). Furthermore, tutorial links are ignoring the alternative text provided, and tutorial links to sections are creating two links: one to the section that uses the URI fragment as the alternative text, and one to the page (that also ignores the alternative text).

This PR applies RebelToolbox/RebelDocumentation#35 to the Rebel Engine API i.e. it improves cross-referencing from the Rebel Engine API to other pages and sections in Rebel Documentation. Links to Rebel Documentation i.e. links to https://docs.rebeltoolbox.com/<lang>/<version>... are created as internal links, which are tested when Rebel Documentation is built.

Links to Rebel Documentation pages are created as:

• :doc:`/full/path/to/page`
• :doc:`Alternative text </full/path/to/page>`

Links to sections in Rebel Documentation pages are created as:

• :ref:`full/path/to/page:section heading`
• :ref:`Alternative text <full/path/to/page:section heading>`

In addition, it uses the same function for:

<tutorials>
    <link title="Alternative text">
        https://docs.rebeltoolbox.com/en/latest/full/path/to/page
    </link>
    <link title="Alternative text">
        https://docs.rebeltoolbox.com/en/latest/full/path/to/page#section-heading
    </link>
</tutorials>

and

<description>
    ... [url=https://docs.rebeltoolbox.com/en/latest/full/path/to/page]
    Alternative text[/url]...
    ... [url=https://docs.rebeltoolbox.com/en/latest/full/path/to/page#section-heading]
    Alternative text[/url]...
</description>

Finally, it improves the look and feel of tutorial links. Specifically:

1. Links are created with the specified alternative text.
2. When referencing a section, only a single link with the specified alternative text is created.

[DanielaOrtner]

Thanks for changing to the correct link format :)