Continuous integration for dev docs

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Implement a continuous integration solution to automatically republish the Development documentation on the project website at www.dita-ot.org/dev whenever changes are pushed to the develop branch of the dita-ot/docs repository.

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The initial CI implementation is available in recent commits to the docs repo.

@jelovirt I suggest we abbreviate the commit SHA in the site repo commit messages to 7 characters, to make the logs a bit more readable:

Deploy dita-ot/docs@65f9630 to dev folder

[jelovirt]

I'd like to make another improvement, where before committing the changes, the script goes through every changed file and checks if only the commit line in YAML front matter has changed. If that's the only change, then we reset that change because we don't actually need changes like that polluting the log.

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Would be great to also run the CI build on pull request branches in addition to develop, but we could treat that as a separate issue.

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The YAML front matter reset in dita-ot/docs@42ab471 and the stable ID generation changes from dita-ot/dita-ot#2276 now yield clean commits that are limited to the changes made in the original source files, like in 92ae1a1.

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Verified successful deployment of changes from dita-ot/docs@d531688 to master in ceb7a02.

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See https://travis-ci.org/dita-ot/docs for latest build status.