Added topic template to `How-to Guides` topic

In the #how-to category, the topic template was changed to the following:

    ## Goal

    To [_do something_].

    ## Assumptions

    You will need:

    * An SSH key configured on your Platform.sh account
    * [_something else_]

    ## Problems

    [_What problem does this how-to guide show you how to solve?_]

    ## Steps

    ### 1. [_Do something as your first step_]

    ```bash
    $ platform init
    $ platform push
    ```

    ### 2. [_Do something else for your second step_]

    [_Instructions about what to do, etc._]

    ## Conclusion

    [_Summary of what was accomplished._]

Now, when a user creates a new topic in this category, the body of the message will automatically be filled with the above to facilitate the user in creating a properly formatted how-to guide.