Deploy user docs only from main MarkBind repository

[Xenonym]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Other, please explain: DevOps enhancement

Closes #749.

What is the rationale for this request?
Our Travis CI configuration will attempt to deploy our user docs from any repository, including forks. This can lead to accidental overwriting of the user docs from developer forks if they have write access to the user docs repository.

What changes did you make? (Give an overview)
I added a on condition to only attempt to deploy user docs when Travis CI runs on the MarkBind/markbind repository. Since this is a common use case, I have also added instructions on how to restrict Travis CI to only deploy from a a specific repository to the user documentation.

Is there anything you'd like reviewers to focus on?
Is the user documentation for restricting Travis CI to only deploy from a a specific repository clear?

Testing instructions:

1. Pull this PR to your own fork, and test with Travis CI enabled. It should not deploy even when on a tagged commit that matches vx.x.x.

Proposed commit message: (wrap lines at 72 characters)

Deploy user docs only from main MarkBind repo

Our Travis CI configuration will attempt to deploy our user docs from
any repository, including forks. This can lead to accidental overwriting
of the user docs from developer forks if they have write access to the
user docs repository.

Let's add a `on` condition to our Travis CI configuration to specify
that deployment of the user docs should only be done when run from the
main MarkBind repository.

Docs: add steps for deploying from specific repo

Restricting Travis CI to only deploy a MarkBind site from a main project
repository should be quite a common use case for projects using
MarkBind.

Let's document how to restrict Travis CI deployment to a specific
repository in our user documentation.

[yamgent]

Thanks for this PR, Travis now no longer complains about deployment errors on my fork. 👍