Skip CI on merging docs-only pull requests

[ashmaroli]

Docs-only pull requests are merged using identifier +doc / +docs.
Jekyllbot should ideally add a [skip ci] to the commit message (preferably as the last line of the extended body) in order to ensure CI is not unnecessarily triggered.

Illustration

Before

$pr_title (#$pr_id)
Merge pull request $pr_id

After

$pr_title (#$pr_id)
Merge pull request $pr_id

[skip ci]