docs: Remove transifex references
<h2>Contribute</h2>
<p>
<a href="{{ pathto('contribute') }}">Ways to Contribute</a> or
- <a href="https://www.transifex.com/projects/p/isso/">Translate</a>
+ <a href="{{ pathto('contribute') }}/#translations">Translate</a>
</p>
{% include "searchbox.html" %}
