[website] file rendering through external binaries (#5387)

* #3758: [doc] file rendering through external binaries

* fix subsections markup

* include proposed changes from PR review

diff --git a/docs/content/doc/advanced/external-renderers.en-us.md b/docs/content/doc/advanced/external-renderers.en-us.md
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+---
+date: "2018-11-23:00:00+02:00"
+title: "External renderers"
+slug: "external-renderers"
+weight: 40
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "External renderers"
+    weight: 40
+    identifier: "external-renderers"
+---
+
+# Custom files rendering configuration
+
+Gitea supports custom file renderings (i.e., Jupyter notebooks, asciidoc, etc.) through external binaries, 
+it is just matter of:
+* installing external binaries
+* add some configuration to your `app.ini` file
+* restart your gitea instance
+
+## Installing external binaries
+
+In order to get file rendering through external binaries, their associated packages must be installed. 
+If you're using a Docker image, your `Dockerfile` should contain something along this lines:
+
+```
+FROM gitea/gitea:1.6.0
+[...]
+
+COPY custom/app.ini /data/gitea/conf/app.ini
+[...]
+
+RUN apk --no-cache add asciidoctor freetype freetype-dev gcc g++ libpng python-dev py-pip python3-dev py3-pip
+# install any other package you need for your external renderers
+
+RUN pip3 install --upgrade pip
+RUN pip3 install -U setuptools
+RUN pip3 install jupyter matplotlib docutils 
+# add above any other python package you may need to install
+```
+
+## `app.ini` file configuration
+
+add one `[markup.XXXXX]` section per external renderer on your custom `app.ini`:
+
+```
+[markup.asciidoc]
+ENABLED = true
+FILE_EXTENSIONS = .adoc,.asciidoc
+RENDER_COMMAND = "asciidoctor --out-file=- -"
+; Input is not a standard input but a file
+IS_INPUT_FILE = false
+
+[markup.jupyter]
+ENABLED = true
+FILE_EXTENSIONS = .ipynb
+RENDER_COMMAND = "jupyter nbconvert --stdout --to html --template basic "
+IS_INPUT_FILE = true
+
+[markup.restructuredtext]
+ENABLED = true
+FILE_EXTENSIONS = .rst
+RENDER_COMMAND = rst2html.py
+IS_INPUT_FILE = false
+```
+
+Once your configuration changes have been made, restart Gitea to have changes take effect.