Travis user docs deployment: restrict deployment to only from `MarkBind/markbind`

[yamgent]

Tell us about your environment

• MarkBind Version: 1.19.2

See #701 (comment).

EDIT: Additionally, developers with travis set up on their fork, with their GITHUB_TOKEN set, should not push any tags that match the vx.x.x format. Unless we have push rights removed from markbind.github.io.git

There is actually a very simple solution to this, by using the repo options:

deploy:
  on:
    repo: MarkBind/markbind

While I have push rights to markbind.github.io, I still don't want my fork to do any deployment (it attempted to do that, but error-ed out because I didn't set GITHUB_TOKEN for my fork). So using the repo option will solve the problem of avoiding deployments on forks.

EDIT: Also, we might also want to include this in the user guide, I think it would be a pretty common use case.