Event Definition: JMTE Template Documentation is unreadable

## Expected Behavior
We provide a link to the JMTE documentation, which should make it easier for a user to create template custom fields.

## Current Behavior
The [link to the docs](https://cdn.jsdelivr.net/gh/DJCordhose/jmte@master/doc/index.html) is delivered with the wrong mime type.

## Possible Solution
We should provide our own JMTE documentation.

## Steps to Reproduce (for bugs)
1. Go to `Fields` in the Event Definition Wizard
2. Add a custom template field
3. Click on the documentation link.

## Context
I was not able to understand the syntax.

## Your Environment
* Graylog Version: 6.1