Added Jekyll template for more readable feature descriptions

[Songtech-0912]

This PR adds a Jekyll template for the feature descriptions, so that what used to be written like this:

<section>
  <div class="features-featurecontainer">
    <div class="features-imagecontainer">
      <img src="https://picsum.photos/1280/720" alt="" class="features-featureimage"/>
    </div>
    <div class="features-textcontainer">
      <h3 class="h3">2D Tracking</h3>
      <p class="p-body">
        Track difficult movements with ease!
        <br />
        <br />
        Natron's tracker can handle and correct a variety of footage types with single
        point or perspective tracking. Luminance variation correction and manual
        keyframes ensure get solid tracks even in tricky shots.
      </p>
    </div>
  </div>
</section>

Becomes the much more easy-to-scan code:

{% include feature-detail.html
   title="2D Tracking"
   img="https://picsum.photos/1280/720"
   img_alt=""
   detail="
    Track difficult movements with ease!
    <br /><br />
    Natron's tracker can handle and correct a variety of footage types with single 
    point or perspective tracking. Luminance variation correction and manual 
    keyframes ensure get solid tracks even in tricky shots."
%}

In addition, the template supports setting the feature description text in markdown so that what previously looked like this:

Natron is compliant with the OpenFX standard, allowing you to tap into the
vast ecosystem of commercial OFX plugins. Never worry about missing the
functionality of your OFX plugins, with Natron's support for <a
href="http://www.revisionfx.com" class="intext-link
intext-link-glow">RevisionFX products</a>, <a
href="https://www.neatvideo.com" class="intext-link
intext-link-glow">NeatVideo denoiser</a>, <a
href="http://www.thefoundry.co.uk/products/furnace/" class="intext-link
intext-link-glow">Furnace by The Foundry</a>, <a
href="http://www.thefoundry.co.uk/products/plugins/keylight/"
class="intext-link intext-link-glow">KeyLight</a>, <a
href="http://www.genarts.com/software/sapphire/overview" class="intext-link
intext-link-glow">GenArts Sapphire</a>, <a
href="http://www.redgiant.com/universe/" class="intext-link
intext-link-glow">Red Giant Universe</a>, <a
href="https://hitfilm.com/ignite" class="intext-link intext-link-glow">Ignite
by HitFilm</a>, and many more!

Now becomes this, which is much-more-readable:

Natron is compliant with the OpenFX standard, allowing you to tap into the
vast ecosystem of commercial OFX plugins. Never worry about missing the
functionality of your OFX plugins, with Natron's support for
[RevisionFX products](http://www.revisionfx.com), [NeatVideo denoiser]
(https://www.neatvideo.com), [Furnace by The Foundry]
(http://www.thefoundry.co.uk/products/furnace/), [KeyLight]
(http://www.thefoundry.co.uk/products/plugins/keylight/), [GenArts Sapphire]
(http://www.genarts.com/software/sapphire/overview), [Red Giant Universe]
(http://www.redgiant.com/universe/), [Ignite by HitFilm]
(https://hitfilm.com/ignite), and many more!

Hope this makes editing the website easier and nicer!

Note: The markdown within the blocks does need to be escaped properly. Specifically, double-quotes must be escaped (e.g. \"...\") and empty newlines are not allowed, instead you should use <br /> to create an effective empty newline.