[FIX] app/website: Gengo doesn't work and can't have worked

Backport of https://github.com/odoo/documentation/pull/1002 .

In V12 and V13, Gengo can't be used on SaaS and Odoo.sh by default
because they don't install the gengo Python library. If the library
is installed on Odoo.sh by the user, it still won't work, because there's
a typo in `base.gengo.translations` here (the -s is missing):
651caf0f59/addons/base_gengo/controller/main.py (L15)

This code is responsible for handling the results returned by Gengo when
the translation is ready. The typo has been present since at least 2016.
Meaning this can't have worked during that period. Fixing that typo
results in the json.loads following it failing due to an unexpected
number of arguments. That code looks it hasn't ran in a long while.

So I propose to remove mention of Gengo from the documentation in V12
and V13 to avoid any future confusion about it possibly working. Since
the feature can't have worked and support for it in V14 has already been
dropped, it doesn't seem worthwhile to attempt to bugfix it at this stage.

content/applications/websites/website/publish/translate.rst

@@ -14,11 +14,7 @@ Process
 Once your website is created, you have the opportunity to translate it
 in as many different languages as you want.
 
-There are two ways to translate your website, you can do it manually or
-automatically with the Gengo App. If you want to do it automatically, go
-to the **App** module and Install **Automated translations through Gengo
-Api** and **Website Gengo Translator**. If you want to do it manually,
-don't install anything, and follow the next step.
+You can only translate your website manually, follow the next step.
 
 Now go to your website. On the bottom right corner of the page, click on
 **Add a language**.
@@ -42,23 +38,3 @@ that some of the text has been translated automatically.
 To translate the content of the website, click on **Translate** (here
 **Traduire** since we want to translate the website in French).
 
-There, if you have installed the Gengo Translator, You will see that
-next to the **Translate** button you also have a button **Translate
-automatically**. Once you click on that button, you will be asked some
-information on your account. If you don't have an account yet, go to
-`*https://gengo.com/auth/form/login/* <https://gengo.com/auth/form/login/>`__
-in order to create one. You need to ask for a public key and a private
-key.
-
-The content you wish to translate will then be translated automatically.
-
-.. image:: media/translate_website04.png
-    :align: center
-
-Now you can see that most of the content is highlighted in yellow or
-in green. The yellow represents the content that you have to translate by
-yourself. The green represents the content that has already been translated
-automatically.
-
-.. image:: media/translate_website05.png
-    :align: center