Use symlink to common

[sandrobonazzola]

Changes proposed in this pull request:

• Using symlinks here to help moving the documentation to upstream first.

I confirm that this pull request was submitted according to the contribution guidelines: @sandrobonazzola

This pull request needs review by: @stoobie @duck-rh

[stoobie]

I fetched the branch, generated ovirt-site and verified that include statements that reference the symlinked /common/ directory work as expected.
I did not verify that all links to the /common/ directory were changes to reference the symlink.

[duck-rh]

I'm all in for upstream first of course \o/.

There's a regression when testing the site with bundle exec jekyll s:

        ** ERROR: directory is already being watched! **

        Directory: /home/duck/Projects/oVirt/ovirt-site/source/documentation/data_warehouse_guide/common

        is already being watched through: /home/duck/Projects/oVirt/ovirt-site/source/documentation/common

and others, so some patterns are not right.

[sandrobonazzola]

I addressed comments about the config file, added a note about the symlink generated error.
I found guard/listen#339 which explain that those errors can be avoided with monkey patching but honestly it's beyond my knowledge of the ruby language.

[duck-rh]

@sandrobonazzola I looked into jekyll-watch and it takes the exclude list into account but skips them if they don't resolve outside the source directory and there is no way to inject custom ignores. I also discovered it does not support globbing like Jekyll does in the ignore field. I then dug into listen and despite uptream's comment on various BRs the ignore list is only skipping events on all architectures (and not just non-linux). Anyway I guess skipping non-directories would still be a problem.

Since there's no way to get the right behavior I looked in the suggested monkeypatch: I believe it is doing the right thing, skipping the warning message but failing in a way that will only skip listening to the symlink target. I have then integrated it in a new patch that I intend to push right after this message.